io_uring: import_single_range() returns 0/-ERROR

Unlike the other core import helpers, import_single_range() returns 0 on
success, not the length imported. This means that links that depend on
the result of non-vec based IORING_OP_{READ,WRITE} that were added for
5.5 get errored when they should not be.

Fixes: 3a6820f2 ("io_uring: add non-vectored read/write commands")
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2075,7 +2075,7 @@ static ssize_t io_import_iovec(int rw, struct io_kiocb *req,
 		ssize_t ret;
 		ret = import_single_range(rw, buf, sqe_len, *iovec, iter);
 		*iovec = NULL;
-		return ret;
+		return ret < 0 ? ret : sqe_len;
 	}
 
 	if (req->io) {