bridge: mdb: Add source VNI support

bridge: mdb: Add source VNI support

In a similar fashion to VXLAN FDB entries, allow user space to program
and view the source VNI of VXLAN MDB entries. Specifically, add support
for the 'MDBE_ATTR_SRC_VNI' and 'MDBA_MDB_EATTR_SRC_VNI' attributes in
request and response messages, respectively.

The source VNI is only relevant when the VXLAN device is in external
mode, where multiple VNIs can be multiplexed over a single VXLAN device.

Example:

 # bridge mdb add dev vxlan0 port vxlan0 grp 239.1.1.1 permanent dst 198.51.100.1 src_vni 2222

 $ bridge -d -s mdb show
 dev vxlan0 port vxlan0 grp 239.1.1.1 permanent filter_mode exclude proto static dst 198.51.100.1 src_vni 2222    0.00

 $ bridge -d -s -j -p mdb show
 [ {
         "mdb": [ {
                 "index": 16,
                 "dev": "vxlan0",
                 "port": "vxlan0",
                 "grp": "239.1.1.1",
                 "state": "permanent",
                 "filter_mode": "exclude",
                 "protocol": "static",
                 "flags": [ ],
                 "dst": "198.51.100.1",
                 "src_vni": 2222,
                 "timer": "   0.00"
             } ],
         "router": {}
     } ]

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