per-tagset SRCU struct and quiesce v3

[Christoph Hellwig]

Hi all,

this series moves the SRCU struct used for quiescing to the tag_set
as the SRCU critical sections for dispatch take about the same time
on all queues anyway, and then adopts the series from Chao that provides
tagset-wide quiesce to use that infrastructure.

To do so this series cleans up a lot of code in nvme that is related
to quiescing queues.

Changes since v2:
 - check blk_queue_skip_tagset_quiesce in blk_mq_unquiesce_tagset
 - drop "nvme-pci: don't warn about the lack of I/O queues for admin
   controllers"
 - drop "nvme: don't call nvme_kill_queues from nvme_remove_namespaces"
 - drop "nvme-pci: mark the namespaces dead earlier in nvme_remove"
 - minor fixups for the above dropped patches
 - fix up a few commit messages

Changes since v1:
 - a whole bunch of new nvme cleanups
 - use the notify version of set_capacity in blk_mark_disk_dead
 - improve the documentation for blk_mq_wait_quiesce_done
 - dynamically allocate the srcu_struct in struct blk_mq_tag_set
 - minor spelling fixes