bridge: Correctly unregister MDB rtnetlink handlers

bridge: Correctly unregister MDB rtnetlink handlers

Commit 6cc129be140b3893ccbe021c66f2482d51053402:
    bridge: Do not unregister all PF_BRIDGE rtnl operations
introduced a bug where a removal of a single bridge from a
multi-bridge system would remove MDB netlink handlers.
The handlers should only be removed once all bridges are gone, but
since we don't keep track of the number of bridge interfaces, it's
simpler to do it when the bridge module is unloaded.  To make it
consistent, move the registration code into module initialization
code path.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>