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

[ Upstream commit c33a39c575068c2ea9bffb22fd6de2df19c74b89 ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>