Support to encoding decoding skb mark on IFE action

Example usage:
Set the skb using skbedit then allow it to be encoded

sudo tc qdisc add dev $ETH root handle 1: prio
sudo tc filter add dev $ETH parent 1: protocol ip prio 10 \
u32 match ip protocol 1 0xff flowid 1:2 \
action skbedit mark 17 \
action ife encode \
allow mark \
dst 02:15:15:15:15:15

Note: You dont need the skbedit action if you are already encoding the
skb mark earlier. A zero skb mark, when seen, will not be encoded.

Alternative hard code static mark of 0x12 every time the filter matches

sudo $TC filter add dev $ETH parent 1: protocol ip prio 10 \
u32 match ip protocol 1 0xff flowid 1:2 \
action ife encode \
type 0xDEAD \
use mark 0x12 \
dst 02:15:15:15:15:15

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>