ipsec: Interfamily IPSec BEET

Here's a revised version, based on Herbert's comments, of a fix for
the ipv6-inner, ipv4-outer interfamily ipsec beet mode. It fixes the
network header adjustment in interfamily, and doesn't reserve space
for the pseudo header anymore when we have ipv6 as the inner family.

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>