Currently this tests tries to umount /sys/kernel/debug (TDIR) but the
system it is running on may have mounts below.

For example, danobi/vmtest [0] VMs have
    mount -t tracefs tracefs /sys/kernel/debug/tracing
as part of their init.

This change instead creates a "random" directory under /tmp and uses this
as TDIR.
If the directory already exists, ignore the error and keep moving on.

Test:

Originally:

    $ vmtest -k $KERNEL_REPO/arch/x86_64/boot/bzImage "./test_progs -vv -a test_bpffs"
    => bzImage
    ===> Booting
    ===> Setting up VM
    ===> Running command
    [    2.138818] bpf_testmod: loading out-of-tree module taints kernel.
    [    2.140913] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    fn:PASS:unshare 0 nsec
    fn:PASS:mount / 0 nsec
    fn:FAIL:umount /sys/kernel/debug unexpected error: -1 (errno 16)
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    test_test_bpffs:PASS:waitpid 0 nsec
    test_test_bpffs:FAIL:bpffs test  failed 255#282     test_bpffs:FAIL
    Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
    Successfully unloaded bpf_testmod.ko.
    Command failed with exit code: 1

After this change:

    $ vmtest -k $(make image_name) 'cd tools/testing/selftests/bpf && ./test_progs -vv -a test_bpffs'
    => bzImage
    ===> Booting
    ===> Setting up VM
    ===> Running command
    [    2.295696] bpf_testmod: loading out-of-tree module taints kernel.
    [    2.296468] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    fn:PASS:unshare 0 nsec
    fn:PASS:mount / 0 nsec
    fn:PASS:mount tmpfs 0 nsec
    fn:PASS:mkdir /tmp/test_bpffs_testdir/fs1 0 nsec
    fn:PASS:mkdir /tmp/test_bpffs_testdir/fs2 0 nsec
    fn:PASS:mount bpffs /tmp/test_bpffs_testdir/fs1 0 nsec
    fn:PASS:mount bpffs /tmp/test_bpffs_testdir/fs2 0 nsec
    fn:PASS:reading /tmp/test_bpffs_testdir/fs1/maps.debug 0 nsec
    fn:PASS:reading /tmp/test_bpffs_testdir/fs2/progs.debug 0 nsec
    fn:PASS:creating /tmp/test_bpffs_testdir/fs1/a 0 nsec
    fn:PASS:creating /tmp/test_bpffs_testdir/fs1/a/1 0 nsec
    fn:PASS:creating /tmp/test_bpffs_testdir/fs1/b 0 nsec
    fn:PASS:create_map(ARRAY) 0 nsec
    fn:PASS:pin map 0 nsec
    fn:PASS:stat(/tmp/test_bpffs_testdir/fs1/a) 0 nsec
    fn:PASS:renameat2(/fs1/a, /fs1/b, RENAME_EXCHANGE) 0 nsec
    fn:PASS:stat(/tmp/test_bpffs_testdir/fs1/b) 0 nsec
    fn:PASS:b should have a's inode 0 nsec
    fn:PASS:access(/tmp/test_bpffs_testdir/fs1/b/1) 0 nsec
    fn:PASS:stat(/tmp/test_bpffs_testdir/fs1/map) 0 nsec
    fn:PASS:renameat2(/fs1/c, /fs1/b, RENAME_EXCHANGE) 0 nsec
    fn:PASS:stat(/tmp/test_bpffs_testdir/fs1/b) 0 nsec
    fn:PASS:b should have c's inode 0 nsec
    fn:PASS:access(/tmp/test_bpffs_testdir/fs1/c/1) 0 nsec
    fn:PASS:renameat2(RENAME_NOREPLACE) 0 nsec
    fn:PASS:access(/tmp/test_bpffs_testdir/fs1/b) 0 nsec
    bpf_testmod.ko is already unloaded.
    Loading bpf_testmod.ko...
    Successfully loaded bpf_testmod.ko.
    test_test_bpffs:PASS:clone 0 nsec
    test_test_bpffs:PASS:waitpid 0 nsec
    test_test_bpffs:PASS:bpffs test  0 nsec
    #282     test_bpffs:OK
    Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
    Successfully unloaded bpf_testmod.ko.

[0] https://github.com/danobi/vmtest

This is a follow-up of https://lore.kernel.org/bpf/20231024201852.1512720-1-chantr4@gmail.com/T/

v1 -> v2:
  - use a TDIR name that is related to test
  - use C-style comments
Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20231031223606.2927976-1-chantr4@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>