Move to blk-mq block interface.

Currently Open-CAS uses single queue block interface,
which is removed in kernel in version 5.0.
From this moment on Open-CAS supports only blk-mq,
and single queue is not supported any longer.

Signed-off-by: Michal Rakowski <michal.rakowski@intel.com>

modules/cas_disk/exp_obj.h

@@ -19,7 +19,6 @@ struct casdsk_exp_obj {
 
 	struct gendisk *gd;
 	struct request_queue *queue;
-	spinlock_t rq_lock;
 
 	struct block_device *locked_bd;