zap_completion_queue() retrieves skbs from completion_queue where they have
zero skb->users counter.  Before dev_kfree_skb_any() it should be non-zero
yet, so it's increased now.
Reported-and-tested-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NJarek Poplawski <jarkao2@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>