Btrfs: Check INCOMPAT flags on remount and add helper function

In support of the recently added capability to remount with lzo
compression, provide a helper function to check the compression
INCOMPAT flags when remounting with lzo compression, and set
the flags if necessary.

Also, implement the new helper function when defragmenting with
explicit lzo compression and when setting the default subvolume.

Signed-off-by: Mitch Harder <mitch.harder@sabayonlinux.org>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>