io_uring/kbuf: shrink nr_iovs/mode in struct buf_sel_arg

nr_iovs is capped at 1024, and mode only has a few low values. We can
safely make them u16, in preparation for adding a few more members.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.h

@@ -56,8 +56,8 @@ struct buf_sel_arg {
 	struct iovec *iovs;
 	size_t out_len;
 	size_t max_len;
-	int nr_iovs;
-	int mode;
+	unsigned short nr_iovs;
+	unsigned short mode;
 };
 
 void __user *io_buffer_select(struct io_kiocb *req, size_t *len,