null_blk: fix type mismatch null_handle_cmd()

In null_handle_cmd() when device is configured as zoned, variable op is
decalred as an int, where it is used to hold values of type
REQ_OP_XXX which is of type enum req_opf. Change the type from
int to enum req_opf.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -1198,7 +1198,7 @@ static blk_status_t null_handle_cmd(struct nullb_cmd *cmd)
 	if (!cmd->error && dev->zoned) {
 		sector_t sector;
 		unsigned int nr_sectors;
-		int op;
+		enum req_opf op;
 
 		if (dev->queue_mode == NULL_Q_BIO) {
 			op = bio_op(cmd->bio);