Fix NULL pointer access in trace_probe_unlink() by initializing
trace_probe.list correctly in trace_probe_init().

In the error case of trace_probe_init(), it can call trace_probe_unlink()
before initializing trace_probe.list member. This causes NULL pointer
dereference at list_del_init() in trace_probe_unlink().

Syzbot reported :

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 8633 Comm: syz-executor797 Not tainted 5.3.0-rc8-next-20190915
#0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:__list_del_entry_valid+0x85/0xf5 lib/list_debug.c:51
Code: 0f 84 e1 00 00 00 48 b8 22 01 00 00 00 00 ad de 49 39 c4 0f 84 e2 00
00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 75
53 49 8b 14 24 4c 39 f2 0f 85 99 00 00 00 49 8d 7d
RSP: 0018:ffff888090a7f9d8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff88809b6f90c0 RCX: ffffffff817c0ca9
RDX: 0000000000000000 RSI: ffffffff817c0a73 RDI: ffff88809b6f90c8
RBP: ffff888090a7f9f0 R08: ffff88809a04e600 R09: ffffed1015d26aed
R10: ffffed1015d26aec R11: ffff8880ae935763 R12: 0000000000000000
R13: 0000000000000000 R14: ffff88809b6f90c0 R15: ffff88809b6f90d0
FS:  0000555556f99880(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000006cc090 CR3: 00000000962b2000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  __list_del_entry include/linux/list.h:131 [inline]
  list_del_init include/linux/list.h:190 [inline]
  trace_probe_unlink+0x1f/0x200 kernel/trace/trace_probe.c:959
  trace_probe_cleanup+0xd3/0x110 kernel/trace/trace_probe.c:973
  trace_probe_init+0x3f2/0x510 kernel/trace/trace_probe.c:1011
  alloc_trace_uprobe+0x5e/0x250 kernel/trace/trace_uprobe.c:353
  create_local_trace_uprobe+0x109/0x4a0 kernel/trace/trace_uprobe.c:1508
  perf_uprobe_init+0x131/0x210 kernel/trace/trace_event_perf.c:314
  perf_uprobe_event_init+0x106/0x1a0 kernel/events/core.c:8898
  perf_try_init_event+0x135/0x590 kernel/events/core.c:10184
  perf_init_event kernel/events/core.c:10228 [inline]
  perf_event_alloc.part.0+0x1b89/0x33d0 kernel/events/core.c:10505
  perf_event_alloc kernel/events/core.c:10887 [inline]
  __do_sys_perf_event_open+0xa2d/0x2d00 kernel/events/core.c:10989
  __se_sys_perf_event_open kernel/events/core.c:10871 [inline]
  __x64_sys_perf_event_open+0xbe/0x150 kernel/events/core.c:10871
  do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Link: http://lkml.kernel.org/r/156869709721.22406.5153754822203046939.stgit@devnote2

Reported-by: syzbot+2f807f4d3a2a4e87f18f@syzkaller.appspotmail.com
Fixes: ca89bc07 ("tracing/kprobe: Add multi-probe per event support")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>