btrfs I/O completion cleanup and single device I/O optimizations v3

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series cleans up the btrfs_bio API, most prominently by splitting
the end_io handler for the highlevel bio from the low-level bio
bi_end_io, which are really confusingly coupled in the current code.
Once that is done it then optimizes the bio submission to not allocate
a btrfs_io_context for I/Os that just go to a single device.

Changes since v2:
 - fix a small comment typo

Changes since v1:
 - add two previously submitted patches skipped from an earlier
   series
 - merged one of those patches with one from this series
 - split one of the patches in this series into three
 - improve various commit logs

Diffstat:
 compression.c    |   43 ++-----
 ctree.h          |    1 
 dev-replace.c    |    5 
 disk-io.c        |   16 +-
 extent-io-tree.h |    4 
 extent_io.c      |  117 +++----------------
 extent_io.h      |    3 
 inode.c          |   57 ++++-----
 raid56.c         |   45 +------
 raid56.h         |    4 
 scrub.c          |    7 -
 super.c          |    6 
 volumes.c        |  337 ++++++++++++++++++++++++++++++++++++++-----------------
 volumes.h        |   20 ++-
 14 files changed, 340 insertions(+), 325 deletions(-)