6LoWPAN: double free in lowpan_fragment_xmit()

dev_queue_xmit() consumes its own skb, so the call to dev_kfree_skb()
in lowpan_fragment_xmit() is a double free.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>