net: bridge: mdb: allow add/delete for host-joined groups

Currently this is needed only for user-space compatibility, so similar
object adds/deletes as the dumped ones would succeed. Later it can be
used for L2 mcast MAC add/delete.

v3: fix compiler warning (DaveM)
v2: don't send a notification when used from user-space, arm the group
    timer if no ports are left after host entry del

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