[IPSEC]: Fix pure tunnel modes involving IPv6

I noticed that my recent patch broke 6-on-4 pure IPsec tunnels (the ones
that are only used for incompressible IPsec packets).  Subsequent reviews
show that I broke 6-on-6 pure tunnels more than three years ago and nobody
ever noticed. I suppose every must be testing 6-on-6 IPComp with large
pings which are very compressible :)

This patch fixes both cases.

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