can: test size of struct sockaddr in sendmsg

This patch makes the CAN socket code conform to the manpage of sendmsg.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>