signal: Prevent double-free of user struct

The way user struct reference counting works changed significantly with,

  fda31c5 ("signal: avoid double atomic counter increments for user accounting")

Now user structs are only freed once the last pending signal is
dequeued. Make sigqueue_free_current() follow this new convention to
avoid freeing the user struct multiple times and triggering this
warning:

 refcount_t: underflow; use-after-free.
 WARNING: CPU: 0 PID: 6794 at lib/refcount.c:288 refcount_dec_not_one+0x45/0x50
 Call Trace:
  refcount_dec_and_lock_irqsave+0x16/0x60
  free_uid+0x31/0xa0
  __dequeue_signal+0x17c/0x190
  dequeue_signal+0x5a/0x1b0
  do_sigtimedwait+0x208/0x250
  __x64_sys_rt_sigtimedwait+0x6f/0xd0
  do_syscall_64+0x72/0x200
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Reported-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/signal.c

@@ -494,8 +494,8 @@ static void sigqueue_free_current(struct sigqueue *q)
 
 	up = q->user;
 	if (rt_prio(current->normal_prio) && !put_task_cache(current, q)) {
-		atomic_dec(&up->sigpending);
-		free_uid(up);
+		if (atomic_dec_and_test(&up->sigpending))
+			free_uid(up);
 	} else
 		  __sigqueue_free(q);
 }