fork: fix pidfd_poll()'s return type

pidfd_poll() is defined as returning 'unsigned int' but the
.poll method is declared as returning '__poll_t', a bitwise type.

Fix this by using the proper return type and using the EPOLL
constants instead of the POLL ones, as required for __poll_t.

Fixes: b53b0b9d ("pidfd: add polling support")
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: stable@vger.kernel.org # 5.3
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191120003320.31138-1-luc.vanoostenryck@gmail.comSigned-off-by: Christian Brauner <christian.brauner@ubuntu.com>

kernel/fork.c

@@ -1708,11 +1708,11 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
 /*
  * Poll support for process exit notification.
  */
-static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts)
+static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 {
 	struct task_struct *task;
 	struct pid *pid = file->private_data;
-	int poll_flags = 0;
+	__poll_t poll_flags = 0;
 
 	poll_wait(file, &pid->wait_pidfd, pts);
 
@@ -1724,7 +1724,7 @@ static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts)
 	 * group, then poll(2) should block, similar to the wait(2) family.
 	 */
 	if (!task || (task->exit_state && thread_group_empty(task)))
-		poll_flags = POLLIN | POLLRDNORM;
+		poll_flags = EPOLLIN | EPOLLRDNORM;
 	rcu_read_unlock();
 
 	return poll_flags;