[IPSEC]: Use the top IPv4 route's peer instead of the bottom

For IPv4 we were using the bottom route's peer instead of the top one.
This is wrong because the peer is only used by TCP to keep track of
information about the TCP destination address which certainly does not
live in the bottom route.

This patch fixes that which allows us to get rid of the family check
since the bottom route could be IPv6 while the top one must always
be IPv4.

I've also changed the other fields which are IPv4-specific to get the
info from the top route instead of potentially bogus data from the
bottom route.

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