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

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

Add flush support to match entries with or without (if "no" is
prepended) added_by_user flag. Note that NTF_USE is used internally
because there is no NTF_ flag that describes such entries.

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

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

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