ptrace: PTRACE_LISTEN forgets to unlock ->siglock

If PTRACE_LISTEN fails after lock_task_sighand() it doesn't drop ->siglock.
Reported-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -744,20 +744,17 @@ int ptrace_request(struct task_struct *child, long request,
 			break;
 
 		si = child->last_siginfo;
-		if (unlikely(!si || si->si_code >> 8 != PTRACE_EVENT_STOP))
-			break;
-
+		if (likely(si && (si->si_code >> 8) == PTRACE_EVENT_STOP)) {
 			child->jobctl |= JOBCTL_LISTENING;
-
 			/*
-		 * If NOTIFY is set, it means event happened between start
-		 * of this trap and now.  Trigger re-trap immediately.
+			 * If NOTIFY is set, it means event happened between
+			 * start of this trap and now.  Trigger re-trap.
 			 */
 			if (child->jobctl & JOBCTL_TRAP_NOTIFY)
 				signal_wake_up(child, true);
-
-		unlock_task_sighand(child, &flags);
 			ret = 0;
+		}
+		unlock_task_sighand(child, &flags);
 		break;
 
 	case PTRACE_DETACH:	 /* detach a process that was attached. */