um: Use enum req_op where appropriate

Improve static type checking by using type enum req_op instead of int where
appropriate.

Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-21-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

arch/um/drivers/ubd_kern.c

@@ -1262,7 +1262,7 @@ static void ubd_map_req(struct ubd *dev, struct io_thread_req *io_req,
 	struct req_iterator iter;
 	int i = 0;
 	unsigned long byte_offset = io_req->offset;
-	int op = req_op(req);
+	enum req_op op = req_op(req);
 
 	if (op == REQ_OP_WRITE_ZEROES || op == REQ_OP_DISCARD) {
 		io_req->io_desc[0].buffer = NULL;
@@ -1325,7 +1325,7 @@ static int ubd_submit_request(struct ubd *dev, struct request *req)
 	int segs = 0;
 	struct io_thread_req *io_req;
 	int ret;
-	int op = req_op(req);
+	enum req_op op = req_op(req);
 
 	if (op == REQ_OP_FLUSH)
 		segs = 0;