nilfs2: add feature set fields to super block

This adds three new fields to nilfs_super_block structure, compatible
feature set, readonly-compatible feature set, and incompatible feature
set in order to prepare for future disk format modifications.

The role of these fields conforms to those of ext3 or other
filesystems.  Most important flags are the incompatible feature set;
it is used to refuse to mount the filesystem which sets an
incompatible feature the kernel doesn't know about.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>