Btrfs: restructure btrfs_run_defrag_inodes()

This patch restructure btrfs_run_defrag_inodes() and make the code of the auto
defragment more readable.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

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

diff --cc fs/btrfs/disk-io.c
Simple merge

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