bridge: mdb: Add underlay destination IP support

bridge: mdb: Add underlay destination IP support

Allow user space to program and view VXLAN MDB entries. Specifically,
add support for the 'MDBE_ATTR_DST' and 'MDBA_MDB_EATTR_DST' attributes
in request and response messages, respectively.

The attributes encode the IP address of the destination VXLAN tunnel
endpoint where multicast receivers for the specified multicast flow
reside.

Multiple destinations can be added for each flow.

Example:

 # bridge mdb add dev vxlan0 port vxlan0 grp 239.1.1.1 permanent dst 198.51.100.1
 # bridge mdb add dev vxlan0 port vxlan0 grp 239.1.1.1 permanent dst 192.0.2.1

 $ bridge -d -s mdb show
 dev vxlan0 port vxlan0 grp 239.1.1.1 permanent filter_mode exclude proto static dst 192.0.2.1    0.00
 dev vxlan0 port vxlan0 grp 239.1.1.1 permanent filter_mode exclude proto static dst 198.51.100.1    0.00

 $ bridge -d -s -j -p mdb show
 [ {
         "mdb": [ {
                 "index": 15,
                 "dev": "vxlan0",
                 "port": "vxlan0",
                 "grp": "239.1.1.1",
                 "state": "permanent",
                 "filter_mode": "exclude",
                 "protocol": "static",
                 "flags": [ ],
                 "dst": "192.0.2.1",
                 "timer": "   0.00"
             },{
                 "index": 15,
                 "dev": "vxlan0",
                 "port": "vxlan0",
                 "grp": "239.1.1.1",
                 "state": "permanent",
                 "filter_mode": "exclude",
                 "protocol": "static",
                 "flags": [ ],
                 "dst": "198.51.100.1",
                 "timer": "   0.00"
             } ],
         "router": {}
     } ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>