posix-timers: Drop signal if timer has been deleted or reprogrammed

No point in delivering a signal from the past. POSIX does not specify the
behaviour here:

 - "The effect of disarming or resetting a timer with pending expiration
    notifications is unspecified."

 - "The disposition of pending signals for the deleted timer is unspecified."

In both cases it is reasonable to expect that pending signals are
discarded. Especially in the reprogramming case it does not make sense to
account for previous overruns or to deliver a signal for a timer which has
been disarmed.

Drop the signal as that is conistent and understandable behaviour.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20241001083835.553646280@linutronix.de