Btrfs: disable snapshot aware defrag for now

commit 8101c8dbf6243ba517aab58d69bf1bc37d8b7b9c upstream.

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,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>