perf tools: Fix trigger class trigger_on()

perf tools: Fix trigger class trigger_on()

trigger_on() means that the trigger is available but not ready, however
trigger_on() was making it ready. That can segfault if the signal comes
before trigger_ready(). e.g. (USR2 signal delivery not shown)

  $ perf record -e intel_pt//u -S sleep 1
  perf: Segmentation fault
  Obtained 16 stack frames.
  /home/ahunter/bin/perf(sighandler_dump_stack+0x40) [0x4ec550]
  /lib/x86_64-linux-gnu/libc.so.6(+0x36caf) [0x7fa76411acaf]
  /home/ahunter/bin/perf(perf_evsel__disable+0x26) [0x4b9dd6]
  /home/ahunter/bin/perf() [0x43a45b]
  /lib/x86_64-linux-gnu/libc.so.6(+0x36caf) [0x7fa76411acaf]
  /lib/x86_64-linux-gnu/libc.so.6(__xstat64+0x15) [0x7fa7641d2cc5]
  /home/ahunter/bin/perf() [0x4ec6c9]
  /home/ahunter/bin/perf() [0x4ec73b]
  /home/ahunter/bin/perf() [0x4ec73b]
  /home/ahunter/bin/perf() [0x4ec73b]
  /home/ahunter/bin/perf() [0x4eca15]
  /home/ahunter/bin/perf(machine__create_kernel_maps+0x257) [0x4f0b77]
  /home/ahunter/bin/perf(perf_session__new+0xc0) [0x4f86f0]
  /home/ahunter/bin/perf(cmd_record+0x722) [0x43c132]
  /home/ahunter/bin/perf() [0x4a11ae]
  /home/ahunter/bin/perf(main+0x5d4) [0x427fb4]

Note, for testing purposes, this is hard to hit unless you add some sleep()
in builtin-record.c before record__open().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: stable@vger.kernel.org
Fixes: 3dcc4436fa6f ("perf tools: Introduce trigger class")
Link: http://lkml.kernel.org/r/1519807144-30694-1-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>