[IPSEC]: Merge most of the input path

As part of the work on asynchronous cryptographic operations, we need
to be able to resume from the spot where they occur.  As such, it
helps if we isolate them to one spot.

This patch moves most of the remaining family-specific processing into
the common input code.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>