net: bridge: vlan: annotate lockless use of msti

Annotate data races around vlan msti field, it can be read lockless.

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260903143138.1574598-8-razor@blackwall.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>