FS/OMFS: block number sanity check during fill_super operation

This patch defines maximum block number to 2^31.  It also converts
bitmap_size and array_size to unsigned int in omfs_get_imap

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Bob Copeland <me@bobcopeland.com>
Tested-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>