sfc: generate encap headers for TC offload

Support constructing VxLAN and GENEVE headers, on either IPv4 or IPv6,
 using the neighbouring information obtained in encap->neigh to
 populate the Ethernet header.
Note that the ef100 hardware does not insert UDP checksums when
 performing encap, so for IPv6 the remote endpoint will need to be
 configured with udp6zerocsumrx or equivalent.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansen-van-vuuren@amd.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>