bridge: vlan: use br_vlan_(get|put)_master to deal with refcounts

Introduce br_vlan_(get|put)_master which take a reference (or create the
master vlan first if it didn't exist) and drop a reference respectively.

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