bridge: mdb: Add get support

bridge: mdb: Add get support

Implement MDB get functionality, allowing user space to query a single
MDB entry from the kernel instead of dumping all the entries. Example
usage:

 # bridge mdb add dev br0 port swp1 grp 239.1.1.1 vid 10
 # bridge mdb add dev br0 port swp2 grp 239.1.1.1 vid 10
 # bridge mdb add dev br0 port swp2 grp 239.1.1.5 vid 10
 # bridge mdb get dev br0 grp 239.1.1.1 vid 10
 dev br0 port swp1 grp 239.1.1.1 temp vid 10
 dev br0 port swp2 grp 239.1.1.1 temp vid 10
 # bridge -j -p mdb get dev br0 grp 239.1.1.1 vid 10
 [ {
         "index": 10,
         "dev": "br0",
         "port": "swp1",
         "grp": "239.1.1.1",
         "state": "temp",
         "flags": [ ],
         "vid": 10
     },{
         "index": 10,
         "dev": "br0",
         "port": "swp2",
         "grp": "239.1.1.1",
         "state": "temp",
         "flags": [ ],
         "vid": 10
     } ]
 # bridge mdb get dev br0 grp 239.1.1.1 vid 20
 Error: bridge: MDB entry not found.
 # bridge mdb get dev br0 grp 239.1.1.2 vid 10
 Error: bridge: MDB entry not found.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David Ahern <dsahern@kernel.org>