signal/posixtimers: Handle ignore/blocked sequences correctly

syzbot triggered the warning in posixtimer_send_sigqueue(), which warns
about a non-ignored signal being already queued on the ignored list.

The warning is actually bogus, as the following sequence causes this:

    signal($SIG, SIGIGN);
    timer_settime(...);			// arm periodic timer

      timer fires, signal is ignored and queued on ignored list

    sigprocmask(SIG_BLOCK, ...);        // block the signal
    timer_settime(...);			// re-arm periodic timer

      timer fires, signal is not ignored because it is blocked
        ---> Warning triggers as signal is on the ignored list

Ideally timer_settime() could remove the signal, but that's racy and
incomplete vs. other scenarios and requires a full reevaluation of the
pending signal list.

Instead of adding more complexity, handle it gracefully by removing the
warning and requeueing the signal to the pending list. That's correct
versus:

  1) sig[timed]wait() as that does not check for SIGIGN and only relies on
     dequeue_signal() -> posixtimers_deliver_signal() to check whether the
     pending signal is still valid.

  2) Unblocking of the signal.

     - If the unblocking happens before SIGIGN is replaced by a signal
       handler, then the timer is rearmed in dequeue_signal(), but
       get_signal() will ignore it. The next timer expiry will move it back
       to the ignored list.

     - If SIGIGN was replaced before unblocking, then the signal will be
       delivered and a subsequent expiry will queue a signal on the pending
       list again.

There is a related scenario to trigger the complementary warning in the
signal ignored path, which does not expect the signal to be on the pending
list when it is ignored. That can be triggered even before the above change
via:

task1			task2

signal($SIG, SIGIGN);
			sigprocmask(SIG_BLOCK, ...);

timer_create();		// Signal target is task2
timer_settime(...);	// arm periodic timer

   timer fires, signal is not ignored because it is blocked
   and queued on the pending list of task2

       	      	     	syscall()
			   // Sets the pending flag
			   sigprocmask(SIG_UNBLOCK, ...);

			-> preemption, task2 cannot dequeue the signal

timer_settime(...);	// re-arm periodic timer

   timer fires, signal is ignored
        ---> Warning triggers as signal is on task2's pending list
	     and the thread group is not exiting

Consequently, remove that warning too and just keep the signal on the
pending list.

The following attempt to deliver the signal on return to user space of
task2 will ignore the signal and a subsequent expiry will bring it back to
the ignored list, if it did not get blocked or un-ignored before that.

Fixes: df7a996b ("signal: Queue ignored posixtimers on ignore list")
Reported-by:  <syzbot+3c2e3cc60665d71de2f7@syzkaller.appspotmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/87ikqhcnjn.ffs@tglx