CVE-2024-40920 net: bridge: mst: fix suspicious rcu usage in br_mst_set_state

In the Linux kernel, the following vulnerability has been resolved:

net: bridge: mst: fix suspicious rcu usage in br_mst_set_state

I converted br_mst_set_state to RCU to avoid a vlan use-after-free
but forgot to change the vlan group dereference helper. Switch to vlan
group RCU deref helper to fix the suspicious rcu usage warning.