dm: add 'blk_mq_nr_hw_queues' and 'blk_mq_queue_depth' module params

dm: add 'blk_mq_nr_hw_queues' and 'blk_mq_queue_depth' module params

Allow user to change these values via module params or sysfs.

'blk_mq_nr_hw_queues' defaults to 1 (max 32).

'blk_mq_queue_depth' defaults to 2048 (up from 64, which proved far too
small under moderate sized workloads -- the dm-multipath device would
continuously block waiting for tags (requests) to become available).
The maximum is BLK_MQ_MAX_DEPTH (currently 10240).

Keep in mind the total number of pre-allocated requests per rq-based DM
blk-mq device is 'blk_mq_nr_hw_queues' * 'blk_mq_queue_depth' (currently
2048).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>