dm crypt: add null iv

Add a new IV generation method 'null' to read old filesystem images created
with SuSE's loop_fish2 module.

Signed-off-by: Ludwig Nussel <ludwig.nussel@suse.de>
Acked-By: Christophe Saout <christophe@saout.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>