nvmet: avoid unneeded assignment of submit_bio return value

We actually using the cookie returned from the last submit_bio
call.
Signed-off-by: NMax Gurtovoy <maxg@mellanox.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>

drivers/nvme/target/io-cmd.c

@@ -85,7 +85,7 @@ static void nvmet_execute_rw(struct nvmet_req *req)
 			bio_set_op_attrs(bio, op, op_flags);
 
 			bio_chain(bio, prev);
-			cookie = submit_bio(prev);
+			submit_bio(prev);
 		}
 
 		sector += sg->length >> 9;