net/funeth: Support UDP segmentation offload

Handle skbs with SKB_GSO_UDP_L4, advertise the offload in features, and
add an ethtool counter for it. Small change to existing TSO code due to
UDP's different header length.

Signed-off-by: Dimitris Michailidis <dmichail@fungible.com>
Link: https://lore.kernel.org/r/20220622223703.59886-1-dmichail@fungible.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>