omfs: sanity check cluster size

A corrupt filesystem could have a bad cluster size; this could result in
the filesystem allocating too much space for files if too large, or
getting stuck in omfs_allocate_block if too small.  The proper range is
1-8 blocks.

Reported-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Bob Copeland <me@bobcopeland.com>