bridge: fdb: add new flush command

bridge: fdb: add new flush command

Add support for fdb bulk delete (aka flush) command. Currently it only
supports the self and master flags with the same semantics as fdb
add/del. The device is a mandatory argument.

Example:
$ bridge fdb flush dev br0
This will delete *all* fdb entries in br0's fdb table.

$ bridge fdb flush dev swp1 master
This will delete all fdb entries pointing to swp1.

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David Ahern <dsahern@kernel.org>