In packet offload, packets are not encrypted in XFRM stack, so
the next network layer which the packets will be forwarded to
should depend on where the packet came from (either xfrm4_output
or xfrm6_output) rather than the matched SA's family type.

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>