RFC: (almost) stop poking into bvec internals in btrfs

[Christoph Hellwig <hch@lst.de>]

Hi Qu,

as you asked for patches to clean up the btrfs poking into bvec
internals, this series does an almost complete sweep.  The only bit
left is the page address in the decompression helper.  I though it
should be able to be replaced with logic based on bbio->file_offset,
but that leads to crashed and data corruption and I ran out of the
time I could allocate to this.

This passes a quick xfstests run with and without -o compress, but
otherwise should be taken as RFC quality code.  Maybe you can take
up the bits you like for your folios work?

Diffstat:
 bio.c         |    9 +-
 bio.h         |    3 
 btrfs_inode.h |    4 -
 compression.c |    9 +-
 disk-io.c     |    7 +-
 extent_io.c   |   60 ++++--------------
 inode.c       |   18 ++---
 raid56.c      |  193 ++++++++++++++++++++++------------------------------------
 scrub.c       |   80 +++++++-----------------
 9 files changed, 142 insertions(+), 241 deletions(-)