atomic queue limits updates

[Christoph Hellwig]

Hi Jens,

currently queue limits updates are a mess in that they are updated one
limit at a time, which makes both cross-checking them against other
limits hard, and also makes it hard to provide atomicy.

This series tries to change this by updating the whole set of queue
limits atomically.   This in done in two ways:

 - for the initial setup the queue_limits structure is simply passed to
   the queue/disk allocation helpers and applies there after validation.
 - for the (relatively few) cases that update limits at runtime a pair
   of helpers to take a snapshot of the current limits and to commit it
   after picking up the callers changes are provided.

As the series is big enough it only converts two drivers - virtio_blk as
a heavily used driver in virtualized setups, and loop as one that actually
does runtime updates while being fairly simple.  I plan to update most
drivers for this merge window, although SCSI will probably have to wait
for the next one given that it will need extensive API changes in the
LLDD and ULD interfaces.

Diffstat:
 arch/um/drivers/ubd_kern.c          |    2 
 block/blk-core.c                    |   29 ++-
 block/blk-mq.c                      |   26 +--
 block/blk-settings.c                |  172 +++++++++++++++++++-
 block/blk-sysfs.c                   |   59 +++----
 block/blk.h                         |    3 
 block/bsg-lib.c                     |    2 
 block/genhd.c                       |    4 
 drivers/block/amiflop.c             |    2 
 drivers/block/aoe/aoeblk.c          |    2 
 drivers/block/ataflop.c             |    2 
 drivers/block/floppy.c              |    2 
 drivers/block/loop.c                |   75 ++++-----
 drivers/block/mtip32xx/mtip32xx.c   |    2 
 drivers/block/nbd.c                 |    2 
 drivers/block/null_blk/main.c       |    2 
 drivers/block/ps3disk.c             |    2 
 drivers/block/rbd.c                 |    2 
 drivers/block/rnbd/rnbd-clt.c       |    2 
 drivers/block/sunvdc.c              |    2 
 drivers/block/swim.c                |    2 
 drivers/block/swim3.c               |    2 
 drivers/block/ublk_drv.c            |    2 
 drivers/block/virtio_blk.c          |  299 ++++++++++++++++++------------------
 drivers/block/xen-blkfront.c        |    2 
 drivers/block/z2ram.c               |    2 
 drivers/cdrom/gdrom.c               |    2 
 drivers/memstick/core/ms_block.c    |    2 
 drivers/memstick/core/mspro_block.c |    2 
 drivers/mmc/core/queue.c            |    2 
 drivers/mtd/mtd_blkdevs.c           |    2 
 drivers/mtd/ubi/block.c             |    2 
 drivers/nvme/host/apple.c           |    2 
 drivers/nvme/host/core.c            |   18 --
 drivers/s390/block/dasd_genhd.c     |    2 
 drivers/s390/block/scm_blk.c        |    2 
 drivers/scsi/scsi_scan.c            |    2 
 drivers/ufs/core/ufshcd.c           |    2 
 include/linux/blk-mq.h              |   10 -
 include/linux/blkdev.h              |   36 +++-
 40 files changed, 484 insertions(+), 305 deletions(-)