crypto: seqiv - Add seqniv

This patch adds a new IV generator seqniv which is identical to
seqiv except that it skips the IV when authenticating.  This is
intended to be used by algorithms such as rfc4106 that does the
IV authentication implicitly.

Note that the code used for seqniv is in fact identical to the
compatibility case for seqiv.

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