bridge: fdb: Add support for locked FDB entries

bridge: fdb: Add support for locked FDB entries

Print the "locked" FDB flag when it is set in the 'NDA_FLAGS_EXT'
attribute. Example output:

 # bridge fdb get 00:11:22:33:44:55 br br0
 00:11:22:33:44:55 dev swp1 locked master br0

 # bridge -j -p fdb get 00:11:22:33:44:55 br br0
 [ {
         "mac": "00:11:22:33:44:55",
         "ifname": "swp1",
         "flags": [ "locked" ],
         "master": "br0",
         "state": ""
     } ]

Signed-off-by: Hans Schultz <netdev@kapio-technology.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>