net: bridge: Extract a helper to handle bridge_binding toggles

Currently, the BROPT_VLAN_BRIDGE_BINDING bridge option is only toggled when
VLAN devices are added on top of a bridge or removed from it. Extract the
toggling of the option to a function so that it could be invoked by a
subsequent patch when the state of an upper VLAN device changes.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/a7455f6fe1dfa7b13126ed8a7fb33d3b611eecb8.1734540770.git.petrm@nvidia.com


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