net/funeth: Unify skb/XDP packet mapping.

Instead of passing an skb to the mapping function pass an
skb_shared_info plus an additional address/length pair. This makes it
usable for both skbs and XDP. Call it from the XDP path and adjust the
skb path.

Signed-off-by: Dimitris Michailidis <dmichail@fungible.com>
Signed-off-by: David S. Miller <davem@davemloft.net>