esp6: Add support for IPsec extended sequence numbers

This patch adds IPsec extended sequence numbers support to esp6.
We use the authencesn crypto algorithm to handle esp with separate
encryption/authentication algorithms.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>