signal: do_signal_stop: Remove the unneeded task_clear_group_stop_pending()

PF_EXITING or TASK_STOPPED has already called task_participate_group_stop()
and cleared its ->group_stop. No need to do task_clear_group_stop_pending()
when we start the new group stop.

Add a small comment to explain the !task_is_stopped() check. Note that this
check is not exactly right and it can lead to unnecessary stop later if the
thread is TASK_PTRACED. What we need is task_participated_in_group_stop(),
this will be solved later.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/signal.c

@@ -1866,7 +1866,8 @@ static int do_signal_stop(int signr)
 		 * still in effect and then receive a stop signal and
 		 * initiate another group stop.  This deviates from the
 		 * usual behavior as two consecutive stop signals can't
-		 * cause two group stops when !ptraced.
+		 * cause two group stops when !ptraced.  That is why we
+		 * also check !task_is_stopped(t) below.
 		 *
 		 * The condition can be distinguished by testing whether
 		 * SIGNAL_STOP_STOPPED is already set.  Don't generate
@@ -1896,8 +1897,6 @@ static int do_signal_stop(int signr)
 				t->group_stop |= signr | gstop;
 				sig->group_stop_count++;
 				signal_wake_up(t, 0);
-			} else {
-				task_clear_group_stop_pending(t);
 			}
 		}
 	}