All common block ciphers have a block size that's a power of 2.  In fact,
all of our block ciphers obey this rule.

If we require this then CBC can be optimised to avoid an expensive divide
on in-place decryption.

I've also changed the saving of the first IV in the in-place decryption
case to the last IV because that lets us use walk->iv (which is already
aligned) for the xor operation where alignment is required.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>