bridge: vlan: don't pass flags when creating context only

We should not pass the original flags when creating a context vlan only
because they may contain some flags that change behaviour in the bridge.
The new global context should be with minimal set of flags, so pass 0
and let br_vlan_add() set the master flag only.

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