Now that libbpf under tools/lib/bpf/* is synced with the version from
samples/bpf, we can get rid most of the libbpf library here.

Committer notes:

Built it in a docker fedora rawhide container and ran it in the f25 host, seems
to work just like it did before this patch, i.e. the switch to tools/lib/bpf/
doesn't seem to have introduced problems and Joe said he tested it with
all the entries in samples/bpf/ and other code he found:

  [root@f5065a7d6272 linux]# make -j4 O=/tmp/build/linux headers_install
  <SNIP>
  [root@f5065a7d6272 linux]# rm -rf /tmp/build/linux/samples/bpf/
  [root@f5065a7d6272 linux]# make -j4 O=/tmp/build/linux samples/bpf/
  make[1]: Entering directory '/tmp/build/linux'
    CHK     include/config/kernel.release
    HOSTCC  scripts/basic/fixdep
    GEN     ./Makefile
    CHK     include/generated/uapi/linux/version.h
    Using /git/linux as source for kernel
    CHK     include/generated/utsrelease.h
    HOSTCC  scripts/basic/bin2c
    HOSTCC  arch/x86/tools/relocs_32.o
    HOSTCC  arch/x86/tools/relocs_64.o
    LD      samples/bpf/built-in.o
  <SNIP>
    HOSTCC  samples/bpf/fds_example.o
    HOSTCC  samples/bpf/sockex1_user.o
  /git/linux/samples/bpf/fds_example.c: In function 'bpf_prog_create':
  /git/linux/samples/bpf/fds_example.c:63:6: warning: passing argument 2 of 'bpf_load_program' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
        insns, insns_cnt, "GPL", 0,
        ^~~~~
  In file included from /git/linux/samples/bpf/libbpf.h:5:0,
                   from /git/linux/samples/bpf/bpf_load.h:4,
                   from /git/linux/samples/bpf/fds_example.c:15:
  /git/linux/tools/lib/bpf/bpf.h:31:5: note: expected 'struct bpf_insn *' but argument is of type 'const struct bpf_insn *'
   int bpf_load_program(enum bpf_prog_type type, struct bpf_insn *insns,
       ^~~~~~~~~~~~~~~~
    HOSTCC  samples/bpf/sockex2_user.o
  <SNIP>
    HOSTCC  samples/bpf/xdp_tx_iptunnel_user.o
  clang  -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/6.2.1/include -I/git/linux/arch/x86/include -I./arch/x86/include/generated/uapi -I./arch/x86/include/generated  -I/git/linux/include -I./include -I/git/linux/arch/x86/include/uapi -I/git/linux/include/uapi -I./include/generated/uapi -include /git/linux/include/linux/kconfig.h  \
	  -D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
	  -Wno-compare-distinct-pointer-types \
	  -Wno-gnu-variable-sized-type-not-at-end \
	  -Wno-address-of-packed-member -Wno-tautological-compare \
	  -O2 -emit-llvm -c /git/linux/samples/bpf/sockex1_kern.c -o -| llc -march=bpf -filetype=obj -o samples/bpf/sockex1_kern.o
    HOSTLD  samples/bpf/tc_l2_redirect
  <SNIP>
    HOSTLD  samples/bpf/lwt_len_hist
    HOSTLD  samples/bpf/xdp_tx_iptunnel
  make[1]: Leaving directory '/tmp/build/linux'
  [root@f5065a7d6272 linux]#

And then, in the host:

  [root@jouet bpf]# mount | grep "docker.*devicemapper\/"
  /dev/mapper/docker-253:0-1705076-9bd8aa1e0af33adce89ff42090847868ca676932878942be53941a06ec5923f9 on /var/lib/docker/devicemapper/mnt/9bd8aa1e0af33adce89ff42090847868ca676932878942be53941a06ec5923f9 type xfs (rw,relatime,context="system_u:object_r:container_file_t:s0:c73,c276",nouuid,attr2,inode64,sunit=1024,swidth=1024,noquota)
  [root@jouet bpf]# cd /var/lib/docker/devicemapper/mnt/9bd8aa1e0af33adce89ff42090847868ca676932878942be53941a06ec5923f9/rootfs/tmp/build/linux/samples/bpf/
  [root@jouet bpf]# file offwaketime
  offwaketime: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=f423d171e0487b2f802b6a792657f0f3c8f6d155, not stripped
  [root@jouet bpf]# readelf -SW offwaketime
  offwaketime         offwaketime_kern.o  offwaketime_user.o
  [root@jouet bpf]# readelf -SW offwaketime_kern.o
  There are 11 section headers, starting at offset 0x700:

  Section Headers:
    [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
    [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
    [ 1] .strtab           STRTAB          0000000000000000 000658 0000a8 00      0   0  1
    [ 2] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  4
    [ 3] kprobe/try_to_wake_up PROGBITS        0000000000000000 000040 0000d8 00  AX  0   0  8
    [ 4] .relkprobe/try_to_wake_up REL             0000000000000000 0005a8 000020 10     10   3  8
    [ 5] tracepoint/sched/sched_switch PROGBITS        0000000000000000 000118 000318 00  AX  0   0  8
    [ 6] .reltracepoint/sched/sched_switch REL             0000000000000000 0005c8 000090 10     10   5  8
    [ 7] maps              PROGBITS        0000000000000000 000430 000050 00  WA  0   0  4
    [ 8] license           PROGBITS        0000000000000000 000480 000004 00  WA  0   0  1
    [ 9] version           PROGBITS        0000000000000000 000484 000004 00  WA  0   0  4
    [10] .symtab           SYMTAB          0000000000000000 000488 000120 18      1   4  8
  Key to Flags:
    W (write), A (alloc), X (execute), M (merge), S (strings)
    I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
    O (extra OS processing required) o (OS specific), p (processor specific)
    [root@jouet bpf]# ./offwaketime | head -3
  qemu-system-x86;entry_SYSCALL_64_fastpath;sys_ppoll;do_sys_poll;poll_schedule_timeout;schedule_hrtimeout_range;schedule_hrtimeout_range_clock;schedule;__schedule;-;try_to_wake_up;hrtimer_wakeup;__hrtimer_run_queues;hrtimer_interrupt;local_apic_timer_interrupt;smp_apic_timer_interrupt;__irqentry_text_start;cpuidle_enter_state;cpuidle_enter;call_cpuidle;cpu_startup_entry;rest_init;start_kernel;x86_64_start_reservations;x86_64_start_kernel;start_cpu;;swapper/0 4
  firefox;entry_SYSCALL_64_fastpath;sys_poll;do_sys_poll;poll_schedule_timeout;schedule_hrtimeout_range;schedule_hrtimeout_range_clock;schedule;__schedule;-;try_to_wake_up;pollwake;__wake_up_common;__wake_up_sync_key;pipe_write;__vfs_write;vfs_write;sys_write;entry_SYSCALL_64_fastpath;;Timer 1
  swapper/2;start_cpu;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule;__schedule;-;---;; 61
  [root@jouet bpf]#
Signed-off-by: Joe Stringer <joe@ovn.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: netdev@vger.kernel.org
Link: https://github.com/joestringer/linux/commit/5c40f54a52b1f437123c81e21873f4b4b1f9bd55.patch
Link: http://lkml.kernel.org/n/tip-xr8twtx7sjh5821g8qw47yxk@git.kernel.org
[ Use -I$(srctree)/tools/lib/ to support out of source code tree builds, as noticed by Wang Nan ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>