fix read repair on compressed extents v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

while looking into the repair code I found that read repair of compressed
extents is current fundamentally broken, in that repair tries to write
the uncompressed data into a corrupted extent during a repair.  This is
demonstrated by the "btrfs: test read repair on a corrupted compressed
extent" test submitted to xfstests.

This series fixes that, but is a bit invaside as it requires both
refactoring of the compression code and changes to the repair code to
not look up the logic address on every repair attempt.  On the plus
side it removes a whole lot of code.

It is based on the for-next branch plus my "btrfs: repair all known bad
mirrors" patch.

Changes since v1:
 - describe the partial revert that happens in patch 1 better in the
   commit log
 - drop a now incorrect comment
 - do not add a prototype for a non-existent function

Diffstat:
 compression.c |  287 ++++++++++++++++------------------------------------------
 compression.h |   11 --
 ctree.h       |    2 
 extent_io.c   |   93 +++++++-----------
 extent_io.h   |    9 -
 inode.c       |   34 +++---
 volumes.h     |    2 
 7 files changed, 146 insertions(+), 292 deletions(-)