iwlwifi: use GFP_KERNEL to allocate Rx SKB memory

Previously we allocate Rx SKB with GFP_ATOMIC flag. This is because we need
to hold a spinlock to protect the two rx_used and rx_free lists operation
in the rxq.

	spin_lock();
	...
	element = rxq->rx_used.next;
	element->skb = alloc_skb(..., GFP_ATOMIC);
	list_del(element);
	list_add_tail(&element->list, &rxq->rx_free);
	...
	spin_unlock();

After spliting the rx_used delete and rx_free insert into two operations,
we don't require the skb allocation in an atomic context any more (the
function itself is scheduled in a workqueue).

	spin_lock();
	...
	element = rxq->rx_used.next;
	list_del(element);
	...
	spin_unlock();
	...
	element->skb = alloc_skb(..., GFP_KERNEL);
	...
	spin_lock()
	...
	list_add_tail(&element->list, &rxq->rx_free);
	...
	spin_unlock();

This patch should fix the "iwlagn: Can not allocate SKB buffers" warning
we see recently.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>