perf: Fix perf_event_comm() vs. exec() assumption

perf: Fix perf_event_comm() vs. exec() assumption

perf_event_comm() assumes that set_task_comm() is only called on
exec(), and in particular that its only called on current.

Neither are true, as Dave reported a WARN triggered by set_task_comm()
being called on !current.

Separate the exec() hook from the comm hook.

Reported-by: Dave Jones <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
[ Build fix. ]
Signed-off-by: Ingo Molnar <[email protected]>