cleanup btrfs bio handling, part 2 v3

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series removes the need to allocate a separate object for I/O
completions for all read and some write I/Os, and reduced the memory
usage of the low-level bios cloned by btrfs_map_bio by using plain bios
instead of the much larger btrfs_bio.

Changes since v2:
 - rebased to the latests misc-next tree
 - fixed an incorrectly assigned bi_end_io handler in the raid56 code

Changes since v1:
 - rebased to the latests misc-next tree
 - don't call destroy_workqueue on NULL workqueues
 - drop a marginal and slightly controversial cleanup to btrfs_map_bio

Diffstat:
 compression.c |   41 +++++-------
 compression.h |    7 +-
 ctree.h       |   14 ++--
 disk-io.c     |  148 +++++-----------------------------------------
 disk-io.h     |   11 ---
 extent_io.c   |   35 ++++-------
 extent_io.h   |    1 
 inode.c       |  162 +++++++++++++++++++--------------------------------
 raid56.c      |  109 +++++++++++++---------------------
 super.c       |   13 ----
 volumes.c     |  184 +++++++++++++++++++++++++++-------------------------------
 volumes.h     |    8 ++
 12 files changed, 262 insertions(+), 471 deletions(-)