opencrypto - Adjust xforms for explicit extra IV
* Some transforms such as XTS and CTR have some private 'IV'. Since our
cryptosoft is now concurrent, we need to keep that private IV per-op
and not per-session as OpenBSD does. We do this by passing in the
local IV to encrypt() and decrypt() functions, as well as reinit(),
which was already getting it.
* We can now also use AES-XTS and AES-CTR in a concurrent environment
without having to have multiple sessions or an explicit key.
* While here, also radically simplify the reinit() for XTS.
* With these changes we can now use aes-xts for dm_target_crypt
successfully.