net/funeth: Tx handling of XDP with fragments.

By now all the functions fun_xdp_tx() calls are shared with the skb path
and thus are fragment-capable. Update fun_xdp_tx(), that up to now has
been passing just one buffer, to check for fragments and call
accordingly.  This makes XDP_TX and ndo_xdp_xmit fragment-capable.

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