io_uring: fix overlapped memcpy in io_req_map_rw()

When io_req_map_rw() is called from io_rw_prep_async(), it memcpy()
iorw->iter into itself. Even though it doesn't lead to an error, such a
memcpy()'s aliasing rules violation is considered to be a bad practise.

Inline io_req_map_rw() into io_rw_prep_async(). We don't really need any
remapping there, so it's much simpler than the generic implementation.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3071,7 +3071,10 @@ static inline int io_rw_prep_async(struct io_kiocb *req, int rw,
 	if (unlikely(ret < 0))
 		return ret;
 
-	io_req_map_rw(req, iov, iorw->fast_iov, &iorw->iter);
+	iorw->bytes_done = 0;
+	iorw->free_iovec = iov;
+	if (iov)
+		req->flags |= REQ_F_NEED_CLEANUP;
 	return 0;
 }