Btrfs: disable snapshot aware defrag for now

It's just broken and it's taking a lot of effort to fix it, so for now just
disable it so people can defrag in peace.  Thanks,

Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>