omfs: add directory routines

Add lookup and directory management routines for OMFS.  The filesystem uses
hashing based on the filename and stores collisions, unordered, in siblings
of files' inode structures.  To support telldir, the current position in
the hash table is encoded in fpos.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>