net: vlan: fix nlmsg size calculation in vlan_get_size()

This patch fixes the calculation of the nlmsg size, by adding the missing
nla_total_size().

Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>