posix-timers: Set k_itimer:: It_signal to NULL on exit()

Technically it's not required to set k_itimer::it_signal to NULL on exit()
because there is no other thread anymore which could lookup the timer
concurrently.

Set it to NULL for consistency sake and add a comment to that effect.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.196462644@linutronix.de

kernel/time/posix-timers.c

@@ -1101,6 +1101,14 @@ static void itimer_delete(struct k_itimer *timer)
 	}
 	list_del(&timer->list);
 
+	/*
+	 * Setting timer::it_signal to NULL is technically not required
+	 * here as nothing can access the timer anymore legitimately via
+	 * the hash table. Set it to NULL nevertheless so that all deletion
+	 * paths are consistent.
+	 */
+	WRITE_ONCE(timer->it_signal, NULL);
+
 	spin_unlock_irqrestore(&timer->it_lock, flags);
 	release_posix_timer(timer, IT_ID_SET);
 }