ipsec: Interfamily IPSec BEET, ipv4-inner ipv6-outer

Here's a revised version, based on Herbert's comments, of a fix for
the ipv4-inner, ipv6-outer interfamily ipsec beet mode. It fixes the
network header adjustment during interfamily, as well as makes sure
that we reserve enough room for the new ipv6 header if we might have
something else as the inner family. Also, the ipv4 pseudo header
construction was added.

Signed-off-by: Joakim Koskela <jookos@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>