net: bridge: fdb: add ndo_fdb_del_bulk

Add a minimal ndo_fdb_del_bulk implementation which flushes all entries.
Support for more fine-grained filtering will be added in the following
patches.

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>