deal with task_work callbacks adding more work

It doesn't matter on normal return to userland path (we'll recheck the
NOTIFY_RESUME flag anyway), but in case of exit_task_work() we'll
need that as soon as we get callbacks capable of triggering more
task_work_add().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/task_work.c

@@ -60,6 +60,7 @@ void task_work_run(void)
 	struct task_struct *task = current;
 	struct callback_head *p, *q;
 
+	while (1) {
 		raw_spin_lock_irq(&task->pi_lock);
 		p = task->task_works;
 		task->task_works = NULL;
@@ -75,4 +76,5 @@ void task_work_run(void)
 			q->func(q);
 			q = p;
 		}
+	}
 }