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

[ Upstream commit 546ceb1d ]

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=9bbe2de1bc9d470eb5fe
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20240609103654.914987-3-razor@blackwall.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>