benet: stricter vxlan offloading check in be_features_check

When VXLAN offloading is enabled, be_features_check() tries to check if
an encapsulated packet is indeed a VXLAN packet. The check is not strict
enough, and considers any UDP-encapsulated ethernet frame with a 8-byte
tunnel header as being VXLAN. Unfortunately, both GENEVE and VXLAN-GPE
have a 8-byte header, so they get through this check.

Force the UDP destination port to be the one that has been offloaded to
hardware.

Without this, GENEVE-encapsulated packets can end up having an incorrect
checksum when both a GENEVE and a VXLAN (offloaded) tunnel are
configured.

This is similar to commit a547224d ("mlx4e: Do not attempt to
offload VXLAN ports that are unrecognized").

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>