net: bridge: Set strict_start_type for br_policy

Set any new attributes added to br_policy to be parsed strictly, to
prevent userspace from passing garbage.

Signed-off-by: Johannes Nixdorf <jnixdorf-oss@avm.de>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20231016-fdb_limit-v5-4-32cddff87758@avm.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>