Btrfs: fix remount vs autodefrag

If we remount the fs to close the auto defragment or make the fs R/O,
we should stop the auto defragment.

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/file.c
Simple merge

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