omfs: add filesystem documentation

These patches add the Optimized MPEG Filesystem, a proprietary filesystem used
by the embedded devices Rio Karma and ReplayTV, which are no longer
manufactured.  This filesystem module enables people to access files on these
devices.

This patch:

OMFS is a proprietary filesystem created for the ReplayTV and also used by the
Rio Karma.  It uses hash tables with unordered, unbounded lists in each bucket
for directories, extents for data blocks, 64-bit addressing for blocks, with
up to 8K blocks (only 2K of a given block is ever used for metadata, so the FS
still works with 4K pages).

Document the filesystem usage and structures.

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>