net: bridge: mdb: dump host-joined entries as well

Currently we dump only the port mdb entries but we can have host-joined
entries on the bridge itself and they should be treated as normal temp
mdbs, they're already notified:
$ bridge monitor all
[MDB]dev br0 port br0 grp ff02::8 temp

The group will not be shown in the bridge mdb output, but it takes 1 slot
and it's timing out. If it's only host-joined then the mdb show output
can even be empty.

After this patch we show the host-joined groups:
$ bridge mdb show
dev br0 port br0 grp ff02::8 temp

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