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.

Fixes: 3a7c1661 ("net: bridge: mst: fix vlan use-after-free")
Reported-by: syzbot+9bbe2de1bc9d470eb5fe@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9bbe2de1bc9d470eb5feSigned-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20240609103654.914987-3-razor@blackwall.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_mst.c

@@ -102,7 +102,7 @@ int br_mst_set_state(struct net_bridge_port *p, u16 msti, u8 state,
 	int err = 0;
 
 	rcu_read_lock();
-	vg = nbp_vlan_group(p);
+	vg = nbp_vlan_group_rcu(p);
 	if (!vg)
 		goto out;