wait_task_zombie: remove unneeded child->signal check

A zombie must have a valid ->signal, we are going to release it and
__exit_signal() starts with BUG_ON(!sig).

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/exit.c

@@ -1201,7 +1201,7 @@ static int wait_task_zombie(struct task_struct *p, int noreap,
 		return 0;
 	}
 
-	if (likely(p->real_parent == p->parent) && likely(p->signal)) {
+	if (likely(p->real_parent == p->parent)) {
 		struct signal_struct *psig;
 		struct signal_struct *sig;