This patch adds support for using seqiv with AEAD algorithms.  This is
useful for those AEAD algorithms that performs authentication before
encryption because the IV generated by the underlying encryption algorithm
won't be available for authentication.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

This generator generates an IV based on a sequence number by xoring it
with a salt.  This algorithm is mainly useful for CTR and similar modes.

This patch also sets it as the default IV generator for ctr.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>