Compatibility of nilfs partitions is now managed with three feature
sets. This changes old compatibility check with revision number so
that it can accept future revisions.
Note that we can stop support of experimental versions of nilfs that
doesn't know the feature sets by incrementing NILFS_CURR
This inserts compatibility check in nilfs_open function so that it
refuses to open the nilfs partition whose super block has incompatible
features that nilfs-utils doesn't know about.
It is verified only when NILFS_OPEN_RAW mode is specified. Moreover,
only incompat feature set is tested because
