sfc: implement encap TSO on EF100

The NIC only needs to know where the headers it has to edit (TCP and
 inner and outer IPv4) are, which fits GSO_PARTIAL nicely.
It also supports non-PARTIAL offload of UDP tunnels, again just
 needing to be told the outer transport offset so that it can edit
 the UDP length field.
(It's not clear to me whether the stack will ever use the non-PARTIAL
 version with the netdev feature flags we're setting here.)

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>