use regular gendisk registration in device mapper

[Christoph Hellwig <hch@lst.de>]

Hi all,

The device mapper code currently has a somewhat odd gendisk registration
scheme where it calls add_disk early, but uses a special flag to skip the
"queue registration", which is a major part of add_disk.  This series
improves the block layer holder tracking to work on an entirely
unregistered disk and thus allows device mapper to use the normal scheme
of calling add_disk when it is ready to accept I/O.

Note that this leads to a user visible change - the sysfs attributes on
the disk and the dm directory hanging off it are not only visible once
the initial table is loaded.  This did not make a different to my testing
using dmsetup and the lvm2 tools.

Diffstat:
 block/Kconfig             |    4 +
 block/Makefile            |    1 
 block/elevator.c          |    1 
 block/genhd.c             |   42 +++++------
 block/holder.c            |  167 ++++++++++++++++++++++++++++++++++++++++++++++
 drivers/md/Kconfig        |    2 
 drivers/md/bcache/Kconfig |    1 
 drivers/md/dm-ioctl.c     |    4 -
 drivers/md/dm-rq.c        |    1 
 drivers/md/dm.c           |   32 +++-----
 fs/block_dev.c            |  145 ---------------------------------------
 include/linux/blk_types.h |    3 
 include/linux/genhd.h     |   19 ++---
 13 files changed, 219 insertions(+), 203 deletions(-)