signals: do_signal_stop(): use signal_group_exit()

do_signal_stop() needs signal_group_exit() but checks sig->group_exit_task.
 This (optimization) is correct, SIGNAL_STOP_DEQUEUED and SIGNAL_GROUP_EXIT
are mutually exclusive, but looks confusing.  Use signal_group_exit(), this
is not fastpath, the code clarity is more important.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -1717,7 +1717,7 @@ static int do_signal_stop(int signr)
 		struct task_struct *t;
 
 		if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
-		    unlikely(sig->group_exit_task))
+		    unlikely(signal_group_exit(sig)))
 			return 0;
 		/*
 		 * There is no group stop already in progress.