This patch replaces the seqniv generator with seqiv when the
underlying algorithm understands the new calling convention.

This not only makes more sense as now seqiv is solely responsible
for IV generation rather than also determining how the IV is going
to be used, it also allows for optimisations in the underlying
implementation.  For example, the space for the IV could be used
to add padding for authentication.

This patch also removes the unnecessary copying of IV to dst
during seqiv decryption as the IV is part of the AD and not cipher
text.

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