io_uring: fix poll error reporting

We should not return an error code in req->result in
io_poll_check_events(), because it may get mangled and returned as
success. Just return the error code directly, the callers will fail the
request or proceed accordingly.

Fixes: 6bf9c47 ("io_uring: defer file assignment")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5f03514ee33324dc811fb93df84aee0f695fb044.1649862516.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5861,9 +5861,8 @@ static int io_poll_check_events(struct io_kiocb *req, bool locked)
 			unsigned flags = locked ? 0 : IO_URING_F_UNLOCKED;
 
 			if (unlikely(!io_assign_file(req, flags)))
-				req->result = -EBADF;
-			else
-				req->result = vfs_poll(req->file, &pt) & req->apoll_events;
+				return -EBADF;
+			req->result = vfs_poll(req->file, &pt) & req->apoll_events;
 		}
 
 		/* multishot, just fill an CQE and proceed */