ofproto: Add NXM_NX_TUN_GBP_ID and NXM_NX_TUN_GBP_FLAGS

ofproto: Add NXM_NX_TUN_GBP_ID and NXM_NX_TUN_GBP_FLAGS

Introduces two new NXMs to represent VXLAN-GBP [0] fields.

  actions=load:0x10->NXM_NX_TUN_GBP_ID[],NORMAL
  tun_gbp_id=0x10,actions=drop

This enables existing VXLAN tunnels to carry security label
information such as a SELinux context to other network peers.

The values are carried to/from the datapath using the attribute
OVS_TUNNEL_KEY_ATTR_VXLAN_OPTS.

[0] https://tools.ietf.org/html/draft-smith-vxlan-group-policy-00

Signed-off-by: Madhu Challa <challa@noironetworks.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>