net: bridge: mcast: dump ipv6 querier state

Add support for dumping global IPv6 querier state, we dump the state
only if our own querier is enabled or there has been another external
querier which has won the election. For the bridge global state we use
a new attribute IFLA_BR_MCAST_QUERIER_STATE and embed the state inside.
The structure is:
  [IFLA_BR_MCAST_QUERIER_STATE]
   `[BRIDGE_QUERIER_IPV6_ADDRESS] - ip address of the querier
   `[BRIDGE_QUERIER_IPV6_PORT]    - bridge port ifindex where the querier
                                    was seen (set only if external querier)
   `[BRIDGE_QUERIER_IPV6_OTHER_TIMER]   -  other querier timeout

IPv4 and IPv6 attributes are embedded at the same level of
IFLA_BR_MCAST_QUERIER_STATE. If we didn't dump anything we cancel the nest
and return.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>