net: bridge: export vlan flags with the stats

Use one of the vlan xstats padding fields to export the vlan flags. This is
needed in order to be able to distinguish between master (bridge) and port
vlan entries in user-space when dumping the bridge vlan stats.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>