enic: add udp_tunnel ndo for vxlan offload

Defines enic_udp_tunnel_add/del for configuring vxlan tunnel offload.
enic supports offload of only one ipv4/udp port.

There are two modes that fw supports for vxlan offload.

mode 0: fcoe bit is set for encapsulated packet. fcoe_fc_crc_ok is set
if checksum of csum is ok. This bit is or of ip_csum_ok and
tcp_udp_csum_ok

mode 2: BIT(0) in rss_hash is set if it is encapsulated packet.
	BIT(1) is set if outer_ip_csum_ok/
	BIT(2) is set if outer_tcp_csum_ok

tcp_udp_csum_ok/ipv4_csum_ok is set if inner csum is OK.

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>