net/funeth: Unify skb/XDP Tx packet unmapping.

Current XDP unmapping is a subset of its skb analog, dealing with
only one buffer. In preparation for multi-frag XDP rename the skb
function and use it also for XDP. The XDP version is removed.

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