posix-timers: Rework timer removal

posix-timers: Rework timer removal

sys_timer_delete() and the do_exit() cleanup function itimer_delete() are
doing the same thing, but have needlessly different implementations instead
of sharing the code.

The other oddity of timer deletion is the fact that the timer is not
invalidated before the actual deletion happens, which allows concurrent
lookups to succeed.

That's wrong because a timer which is in the process of being deleted
should not be visible and any actions like signal queueing, delivery and
rearming should not happen once the task, which invoked timer_delete(), has
the timer locked.

Rework the code so that:

   1) The signal queueing and delivery code ignore timers which are marked
      invalid

   2) The deletion implementation between sys_timer_delete() and
      itimer_delete() is shared

   3) The timer is invalidated and removed from the linked lists before
      the deletion callback of the relevant clock is invoked.

      That requires to rework timer_wait_running() as it does a lookup of
      the timer when relocking it at the end. In case of deletion this
      lookup would fail due to the preceding invalidation and the wait loop
      would terminate prematurely.

      But due to the preceding invalidation the timer cannot be accessed by
      other tasks anymore, so there is no way that the timer has been freed
      after the timer lock has been dropped.

      Move the re-validation out of timer_wait_running() and handle it at
      the only other usage site, timer_settime().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/87zfht1exf.ffs@tglx