vxlan: Fix to enable UDP checksums on interface

Add definition to vxlan nla_policy for UDP checksum. This is necessary
to enable UDP checksums on VXLAN.

In some instances, enabling UDP checksums can improve performance on
receive for devices that return legacy checksum-unnecessary for UDP/IP.
Also, UDP checksum provides some protection against VNI corruption.

Testing:

Ran 200 instances of TCP_STREAM and TCP_RR on bnx2x.

TCP_STREAM
  IPv4, without UDP checksums
      14.41% TX CPU utilization
      25.71% RX CPU utilization
      9083.4 Mbps
  IPv4, with UDP checksums
      13.99% TX CPU utilization
      13.40% RX CPU utilization
      9095.65 Mbps

TCP_RR
  IPv4, without UDP checksums
      94.08% TX CPU utilization
      156/248/462 90/95/99% latencies
      1.12743e+06 tps
  IPv4, with UDP checksums
      94.43% TX CPU utilization
      158/250/462 90/95/99% latencies
      1.13345e+06 tps

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