netpoll: zap_completion_queue: adjust skb->users counter

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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>