io_uring: simplify io_req_map_rw()

Don't deref req->io->rw every time, but put it in a local variable. This
looks prettier, generates less instructions, and doesn't break alias
analysis.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2828,15 +2828,17 @@ static void io_req_map_rw(struct io_kiocb *req, ssize_t io_size,
 			  struct iovec *iovec, struct iovec *fast_iov,
 			  struct iov_iter *iter)
 {
-	req->io->rw.nr_segs = iter->nr_segs;
-	req->io->rw.size = io_size;
-	req->io->rw.iov = iovec;
-	if (!req->io->rw.iov) {
-		req->io->rw.iov = req->io->rw.fast_iov;
-		if (req->io->rw.iov != fast_iov)
-			memcpy(req->io->rw.iov, fast_iov,
+	struct io_async_rw *rw = &req->io->rw;
+
+	rw->nr_segs = iter->nr_segs;
+	rw->size = io_size;
+	if (!iovec) {
+		rw->iov = rw->fast_iov;
+		if (rw->iov != fast_iov)
+			memcpy(rw->iov, fast_iov,
 			       sizeof(struct iovec) * iter->nr_segs);
 	} else {
+		rw->iov = iovec;
 		req->flags |= REQ_F_NEED_CLEANUP;
 	}
 }