netpoll: Use skb_irq_freeable to make zap_completion_queue safe.

Replace the test in zap_completion_queue to test when it is safe to
free skbs in hard irq context with skb_irq_freeable ensuring we only
free skbs when it is safe, and removing the possibility of subtle
problems.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>