blk-mq: initial support for multiple queue maps

Add a queue offset to the tag map. This enables users to map
iteratively, for each queue map type they support.

Bump maximum number of supported maps to 2, we're now fully
able to support more than 1 map.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -78,10 +78,11 @@ struct blk_mq_hw_ctx {
 struct blk_mq_queue_map {
 	unsigned int *mq_map;
 	unsigned int nr_queues;
+	unsigned int queue_offset;
 };
 
 enum {
-	HCTX_MAX_TYPES = 1,
+	HCTX_MAX_TYPES = 2,
 };
 
 struct blk_mq_tag_set {