net: copy_rtnl_link_stats64() simplification

No need to use a temporary struct rtnl_link_stats64 variable,
just copy the source to skb buffer.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>