bridge: vlan: use synchronize_net() when holding RTNL

br_vlan_flush() and nbp_vlan_flush() should use synchronize_net()
instead of syncronize_rcu() to release RTNL sooner.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>