events: Add note to update_event_times comment about holding ctx->lock

Signed-off-by: Eric B Munson <emunson@mgebm.net>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1308861279-15216-1-git-send-email-emunson@mgebm.net


Signed-off-by: Ingo Molnar <mingo@elte.hu>