Make dsmark work properly with non-linear and cloned skb's
Before modifying the header, it needs to check that skb header is
writeable.

Note: this makes the assumption, that if it queues a good skb
then a good skb will come out of the embedded qdisc.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>