blk-mq: Use pointers for blk_mq_tags bitmap tags

blk-mq: Use pointers for blk_mq_tags bitmap tags

Introduce pointers for the blk_mq_tags regular and reserved bitmap tags,
with the goal of later being able to use a common shared tag bitmap across
all HW contexts in a set.

Signed-off-by: John Garry <[email protected]>
Tested-by: Don Brace<[email protected]> #SCSI resv cmds patches used
Tested-by: Douglas Gilbert <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>