net: fix vlan_get_size to include vlan_flags size

Fix vlan_get_size to include vlan->flags.  Currently, the
size of the vlan flags is not included in the nlmsg size.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>