bridge: fdb: add flush port matching

bridge: fdb: add flush port matching

Usually we match on the device specified after "dev" but there are
special cases where we need an additional device attribute for matching
such as when matching entries specifically pointing to the bridge device
itself. We use NDA_IFINDEX for that purpose.

Example:
$ bridge fdb flush dev br0 brport br0
This will flush only entries pointing to the bridge itself.

$ bridge fdb flush dev swp1 brport swp2 master
Note this will flush entries pointing to swp2 only. The NDA_IFINDEX
attribute overrides the dev argument. This is documented in the man
page.

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