io_uring: terminate manual loop iterator loop correctly for non-vecs

The fix for not advancing the iterator if we're using fixed buffers is
broken in that it can hit a condition where we don't terminate the loop.
This results in io-wq looping forever, asking to read (or write) 0 bytes
for every subsequent loop.
Reported-by: Joel Jaeschke <joel.jaeschke@gmail.com>
Link: https://github.com/axboe/liburing/issues/549
Fixes: 16c8d2df ("io_uring: ensure symmetry in handling iter types in loop_rw_iter()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3612,13 +3612,15 @@ static ssize_t loop_rw_iter(int rw, struct io_kiocb *req, struct iov_iter *iter)
 				ret = nr;
 			break;
 		}
+		ret += nr;
 		if (!iov_iter_is_bvec(iter)) {
 			iov_iter_advance(iter, nr);
 		} else {
-			req->rw.len -= nr;
 			req->rw.addr += nr;
+			req->rw.len -= nr;
+			if (!req->rw.len)
+				break;
 		}
-		ret += nr;
 		if (nr != iovec.iov_len)
 			break;
 	}