perf/core: Fix perf_event_read_value() locking

perf_event_read_value() is an external accessor, just like
perf_event_{en,dis}able() and should thus use perf_event_ctx_lock().

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>
Fixes: f63a8daa ("perf: Fix event->ctx locking")
Signed-off-by: Ingo Molnar <mingo@kernel.org>