perf/core: Explain perf_sched_mutex

perf/core: Explain perf_sched_mutex

To clarify why atomic_inc_return(&perf_sched_events) is not sufficient and
a mutex is needed to order static branch enabling vs the atomic counter
increment, this adds a comment with a short explanation.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170829140103.6563-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>