bridge: add sysfs hook to flush forwarding table

The RSTP daemon needs to be able to flush all dynamic forwarding
entries in the case of topology change.

This is a temporary interface. It will change to a netlink interface
before RSTP daemon is officially released.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>

diff --cc net/bridge/br_fdb.c
Simple merge

diff --cc net/bridge/br_private.h
Simple merge

diff --cc net/bridge/br_sysfs_br.c
Simple merge

diff --cc net/bridge/br_sysfs_if.c
Simple merge