bridge: provide lockless access to p->path_cost

bridge: provide lockless access to p->path_cost

Add READ_ONCE()/WRITE_ONCE() annotations around p->path_cost.

This is needed at least for sysfs show_path_cost(), BRCTL_GET_PORT_INFO
and upcoming RTNL avoidance in "ip link" dumps (cf br_port_fill_attrs()).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260604141343.2124500-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>