bridge: fdb: add flush [no]offloaded entry matching

bridge: fdb: add flush [no]offloaded entry matching

Add flush support to match entries with or without (if "no" is
prepended) offloaded flag.

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

$ bridge fdb flush dev br0 nooffloaded
This will delete all entries except the ones with offloaded flag in
br0's fdb table.

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