bridge: mcast: add router port on PIM hello message

When we receive a PIM Hello message on a port we can consider that it
has a multicast router attached, thus it is correct to add it to the
router list. The only catch is it shouldn't be considered for a querier.

Using Daniel's description:
leaf-11  leaf-12  leaf-13
       \   |    /
        bridge-1
         /    \
    host-11  host-12

 - all ports in bridge-1 are in a single vlan aware bridge
 - leaf-11 is the IGMP querier
 - leaf-13 is the PIM DR
 - host-11 TXes packets to 226.10.10.10
 - bridge-1 only forwards the 226.10.10.10 traffic out the port to
   leaf-11, it should also forward this traffic out the port to leaf-13

Suggested-by: Daniel Walton <dwalton@cumulusnetworks.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>