Btrfs iomap

This is an effort to use iomap for btrfs. This would keep most
responsibility of page handling during writes in iomap code, hence
code reduction. For CoW support, changes are needed in iomap code
to make sure we perform a copy before the write.
This is in line with the discussion we had during adding dax support in
btrfs.

Efforts on adding dax support have been put on hold until MM experts can
come with a way of performing multiple mappings to a single page
(primarily the TODO before dax_associate_entry()). While we are waiting
on that we could add support for buffered writes in btrfs.

[1] https://github.com/goldwynr/linux/tree/btrfs-iomap

-- 
Goldwyn

Changes since v1
- Added Direct I/O support
- Remove PagePrivate from btrfs pages for regular files

 fs/btrfs/Makefile           |    2 
 fs/btrfs/compression.c      |    1 
 fs/btrfs/ctree.h            |   15 -
 fs/btrfs/extent_io.c        |   13 
 fs/btrfs/extent_io.h        |    2 
 fs/btrfs/file.c             |  520 --------------------------------------
 fs/btrfs/free-space-cache.c |    1 
 fs/btrfs/inode.c            |  170 +++---------
 fs/btrfs/ioctl.c            |    4 
 fs/btrfs/iomap.c            |  600 +++++++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/relocation.c       |    2 
 fs/dax.c                    |    8 
 fs/ext2/inode.c             |    2 
 fs/ext4/inode.c             |    2 
 fs/gfs2/bmap.c              |    3 
 fs/iomap/apply.c            |    5 
 fs/iomap/buffered-io.c      |   28 +-
 fs/iomap/direct-io.c        |   18 -
 fs/iomap/fiemap.c           |    4 
 fs/iomap/seek.c             |    4 
 fs/iomap/swapfile.c         |    3 
 fs/xfs/xfs_iomap.c          |    9 
 include/linux/iomap.h       |    7 
 23 files changed, 727 insertions(+), 696 deletions(-)