selinux: Fix packet forwarding checks on postrouting

The IPSKB_FORWARDED and IP6SKB_FORWARDED flags are used only in the
multicast forwarding case to indicate that a packet looped back after
forward. So these flags are not a good indicator for packet forwarding.
A better indicator is the incoming interface. If we have no socket context,
but an incoming interface and we see the packet in the ip postroute hook,
the packet is going to be forwarded.

With this patch we use the incoming interface as an indicator on packet
forwarding.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: Eric Paris <eparis@redhat.com>