blk-mq: Register <dev>/queue/mq after having registered <dev>/queue

blk-mq: Register <dev>/queue/mq after having registered <dev>/queue

A later patch in this series will modify blk_mq_debugfs_register()
such that it uses q->kobj.parent to determine the name of a
request queue. Hence make sure that that pointer is initialized
before blk_mq_debugfs_register() is called. To avoid lock inversion,
protect sysfs / debugfs registration with the queue sysfs_lock
instead of the global mutex all_q_mutex.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
[sw0312.kim: backport mainline commit 2d0364c8c1a9 to fix possible deadlock in blk-mq for cpu hotplug]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: Id43aae4f67f6aa43b5ad0ccd19b8f40baab337b8