can: dev: fix nlmsg size calculation in can_get_size()

[ Upstream commit fe119a05f8ca481623a8d02efcc984332e612528 ]

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

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>