be2net: Implement ndo_gso_check()

Use vxlan_gso_check() to advertise offload support for this NIC.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Sathya Perla <sperla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>