alarmtimer: Lock k_itimer during timer callback

alarmtimer: Lock k_itimer during timer callback

Locks the k_itimer's it_lock member when handling the alarm timer's
expiry callback.

The regular posix timers defined in posix-timers.c have this lock held
during timout processing because their callbacks are routed through
posix_timer_fn().  The alarm timers follow a different path, so they
ought to grab the lock somewhere else.

Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Richard Cochran <[email protected]>
Cc: Prarit Bhargava <[email protected]>
Cc: Sharvil Nanavati <[email protected]>
Signed-off-by: Richard Larocque <[email protected]>
Signed-off-by: John Stultz <[email protected]>