blk-mq: Shared tag enhancements

blk-mq: Shared tag enhancements

Orabug: 23521058

Storage controllers may expose multiple block devices that share hardware
resources managed by blk-mq. This patch enhances the shared tags so a
low-level driver can access the shared resources not tied to the unshared
h/w contexts. This way the LLD can dynamically add and delete disks and
request queues without having to track all the request_queue hctx's to
iterate outstanding tags.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit f26cdc8536ad50fb802a0445f836b4f94ca09ae7)
Signed-off-by: Dan Duval <dan.duval@oracle.com>