posix-timers: Ensure that timer initialization is fully visible

Frederic pointed out that the memory operations to initialize the timer are
not guaranteed to be visible, when __lock_timer() observes timer::it_signal
valid under timer::it_lock:

  T0                                      T1
  ---------                               -----------
  do_timer_create()
      // A
      new_timer->.... = ....
      spin_lock(current->sighand)
      // B
      WRITE_ONCE(new_timer->it_signal, current->signal)
      spin_unlock(current->sighand)
					sys_timer_*()
					   t =  __lock_timer()
						  spin_lock(&timr->it_lock)
						  // observes B
						  if (timr->it_signal == current->signal)
						    return timr;
			                   if (!t)
					       return;
					// Is not guaranteed to observe A

Protect the write of timer::it_signal, which makes the timer valid, with
timer::it_lock as well. This guarantees that T1 must observe the
initialization A completely, when it observes the valid signal pointer
under timer::it_lock. sighand::siglock must still be taken to protect the
signal::posix_timers list.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20250308155623.507944489@linutronix.de