Btrfs: early work to file_write in big extents

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --cc fs/btrfs/ctree.c
Simple merge

diff --cc fs/btrfs/ctree.h
Simple merge

diff --cc fs/btrfs/file-item.c
Simple merge

diff --cc fs/btrfs/super.c
Simple merge