btrfs: sysfs: don't show reserved incompat feature

The COMPRESS_LZOv2 incompat featue is currently not implemented, the bit
is only reserved, no point to list it in sysfs.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>