[dm-devel] fix delayed holder tracking v2

[Christoph Hellwig]

Hi all,

this series tries to fix the delayed holder tracking that is only used by
dm by moving it into dm, where we can track the lifetimes much better.

Changes since v1: 
 - don't blow away ->bd_holder_dir in del_gendisk or add_disk failure
   as the holder unregistration references it
 - add an extra cleanup patch

Diffstat:
 block/genhd.c          |    6 --
 block/holder.c         |   85 ++++++++++------------------------
 drivers/md/dm.c        |  122 ++++++++++++++++++++++++++-----------------------
 include/linux/blkdev.h |    5 --
 4 files changed, 93 insertions(+), 125 deletions(-)