do_wait: kill the old BUG_ON, use while_each_thread()

do_wait() does BUG_ON(tsk->signal != current->signal), this looks like a
raher obsolete check.  At least, I don't think do_wait() is the best place
to verify that all threads have the same ->signal.  Remove it.

Also, change the code to use while_each_thread().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: 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

@@ -1595,9 +1595,7 @@ static long do_wait(struct wait_opts *wo)
 
 		if (wo->wo_flags & __WNOTHREAD)
 			break;
-		tsk = next_thread(tsk);
-		BUG_ON(tsk->signal != current->signal);
-	} while (tsk != current);
+	} while_each_thread(current, tsk);
 	read_unlock(&tasklist_lock);
 
 notask: