timerfd: Reject ALARM timerfds without CAP_WAKE_ALARM

timerfd: Reject ALARM timerfds without CAP_WAKE_ALARM

timerfd gives processes a way to set wake alarms, but unlike timers made using
timer_create, timerfds don't check whether the process has CAP_WAKE_ALARM
before setting alarm-time timers. CAP_WAKE_ALARM is supposed to gate this
behavior and so it makes sense that we should deny permission to create such
timerfds if the process doesn't have this capability.

Signed-off-by: Eric Caruso <ejcaruso@google.com>
Cc: Todd Poynor <toddpoynor@google.com>
Link: http://lkml.kernel.org/r/1465427339-96209-1-git-send-email-ejcaruso@chromium.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>