[BLOCK] dm-crypt: trivial comment improvements

Just some minor comment nits.

- little-endian is better than low-endian
- and since it is called essiv everywere it should also be essiv
  in the comments (and not ess_iv)

Signed-off-by: Rik Snel <rsnel@cube.dyndns.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>