cleanup btrfs bio handling, part 2

[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.

Note that this series only applies to the for-next tree, as the misc-next
tree seems to miss the workqueue cleanup series.

Diffstat:
 compression.c |   41 +++++------
 compression.h |    7 +-
 ctree.h       |   14 ++--
 disk-io.c     |  144 +++--------------------------------------
 disk-io.h     |   11 ---
 extent_io.c   |   33 +++------
 extent_io.h   |    1 
 inode.c       |  162 ++++++++++++++++++----------------------------
 raid56.c      |  111 ++++++++++++-------------------
 super.c       |   13 ---
 volumes.c     |  203 ++++++++++++++++++++++++++++------------------------------
 volumes.h     |    8 ++
 12 files changed, 271 insertions(+), 477 deletions(-)