This patch adds a new AEAD IV generator echainiv.  It is intended
to replace the existing skcipher IV generator eseqiv.

If the underlying AEAD algorithm is using the old AEAD interface,
then echainiv will simply use its IV generator.

Otherwise, echainiv will encrypt a counter just like eseqiv but
it'll first xor it against a previously stored IV similar to
chainiv.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>