vxlan: change vxlan to use UDP socket GRO

Adapt vxlan_gro_receive, vxlan_gro_complete to take a socket argument.
Set these functions in tunnel_config.  Don't set udp_offloads any more.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>