xfrm: fix xfrm child route lookup for packet offload

In current code, xfrm_bundle_create() always uses the matched
SA's family type to look up a xfrm child route for the skb.
The route returned by xfrm_dst_lookup() will eventually be
used in xfrm_output_resume() (skb_dst(skb)->ops->local_out()).

If packet offload is used, the above behavior can lead to
calling ip_local_out() for an IPv6 packet or calling
ip6_local_out() for an IPv4 packet, which is likely to fail.

This change fixes the behavior by checking if the matched SA
has packet offload enabled. If not, keep the same behavior;
if yes, use the matched SP's family type for the lookup.

Test: verified IPv6-in-IPv4 packets on Android device with
      IPsec packet offload enabled
Signed-off-by: Mike Yu <yumike@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>