sctp: add skb_sdif in struct sctp_af

Add skb_sdif function in struct sctp_af to get the enslaved device
for both ipv4 and ipv6 when adding SCTP VRF support in sctp_rcv in
the next patch.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>