Btrfs: make ordered operations be handled by multi-task

The process of the ordered operations is similar to the delalloc inode flush, so
we handle them by flush workers.

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

diff --cc fs/btrfs/ordered-data.c
Simple merge

diff --cc fs/btrfs/ordered-data.h
Simple merge

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