bridge: vlan: Use rcu_dereference instead of rtnl_dereference

br_should_learn() is protected by RCU and not by RTNL, so use correct
flavor of nbp_vlan_group().

Fixes: 907b1e6e ("bridge: vlan: use proper rcu for the vlgrp
member")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>