This eighth patch of eight in this cleancache series "opts-in"
cleancache for ocfs2. Clustered filesystems must explicitly enable
cleancache by calling cleancache_init_shared_fs anytime an instance
of the filesystem is mounted. Ocfs2 is currently the only user of
the clustered filesystem interface but nevertheless, the cleancache
hooks in the VFS layer are sufficient for ocfs2 including the matching
cleancache_flush_fs hook which must be called on unmount.
Details and a FAQ can be found in Documentation/vm/cleancache.txt