Btrfs: don't auto defrag a file when doing directIO

If we runt the direct IO, we should not run auto defrag, because it may
introduce buffered IO vs direcIO problem, and make direct IO slow down.

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