vlan: Change vlan_dev_set_vlan_flag() to handle multiple flags at once

Change vlan_dev_set_vlan_flag() to handle multiple flags at once and
rename to vlan_dev_change_flags(). This allows to to use it from the
netlink interface, which in turn allows to handle necessary adjustments
when changing flags centrally.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>