io_uring: do not always copy iovec in io_req_map_rw()

In io_read_prep() or io_write_prep(), io_req_map_rw() takes
struct io_async_rw's fast_iov as argument to call io_import_iovec(),
and if io_import_iovec() uses struct io_async_rw's fast_iov as
valid iovec array, later indeed io_req_map_rw() does not need
to do the memcpy operation, because they are same pointers.
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2493,6 +2493,7 @@ static void io_req_map_rw(struct io_kiocb *req, ssize_t 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,
 			       sizeof(struct iovec) * iter->nr_segs);
 	} else {