fix queue freeze and limit locking order v3

[Christoph Hellwig]

Hi all,

this is my version of Damien's "Fix queue freeze and limit locking order".
A lot looks very similar, but it was done independently based on the
previous discussion.

Changes since v2:
 - check for polling support under q_usage_counter
 - improve a commit log

Changes since v1:
 - more comment typo fixing
 - fix loop as well
 - make the poll sysfs attr show method more accurate
 
Changes since RFC:
 - fix a bizzare virtio_blk bisection snafu
 - set BLK_FEAT_POLL a little less eagerly for blk-mq
 - drop the loop patch just adding a comment
 - improve various commit logs and coments

Diffstat:
 block/blk-core.c               |   17 ++++-
 block/blk-integrity.c          |    4 -
 block/blk-mq.c                 |   17 -----
 block/blk-settings.c           |   27 +++++++-
 block/blk-sysfs.c              |  134 ++++++++++++++++++++---------------------
 block/blk-zoned.c              |    7 --
 block/blk.h                    |    1 
 drivers/block/loop.c           |   52 ++++++++++-----
 drivers/block/nbd.c            |   17 -----
 drivers/block/virtio_blk.c     |    4 -
 drivers/nvme/host/core.c       |    9 +-
 drivers/scsi/sd.c              |   17 +----
 drivers/scsi/sr.c              |    5 -
 drivers/usb/storage/scsiglue.c |    5 -
 include/linux/blkdev.h         |    5 -
 15 files changed, 164 insertions(+), 157 deletions(-)