bridge: vlan: add support for mcast_router option

bridge: vlan: add support for mcast_router option

Add support for setting and dumping per-vlan/interface mcast_router
option. It controls the mcast router mode of a vlan/interface pair.
For bridge devices only modes 0 - 2 are allowed. The possible modes
are:
 0 - disabled
 1 - automatic router presence detection (default)
 2 - permanent router
 3 - temporary router (available only for ports)

Example:
 # mark port ens16 as a permanent mcast router for vlan 100
 $ bridge vlan set dev ens16 vid 100 mcast_router 2
 # disable mcast router for port ens16 and vlan 200
 $ bridge vlan set dev ens16 vid 200 mcast_router 0
 $ bridge -d vlan show
 port              vlan-id
 ens16             1 PVID Egress Untagged
                     state forwarding mcast_router 1
                   100
                     state forwarding mcast_router 2
                   200
                     state forwarding mcast_router 0

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>