Backport multiqueue support for kernels < 2.6.27

The 2.6.23 kernel added some initial multiqueue support.
That release relied on the on the notion of struct
net_device_subqueue attached to the netdevice struct
as an array. The 2.6.27 renamed these to struct netdev_queue,
and enhanced MQ support by providing locks separately onto
each queue. MQ support on 2.6.27 also extended each netdev
to be able to assign a select_queue callback to be used by
core networking for prior to pushing the skb out to the device
driver so that queue selection can be dealt with and
customized internally on the driver.

For kernels 2.6.23..2.6.26 then we backport MQ support by
using the equivalent calls on the struct netdev_queue to
the struct net_device_subqueue. The performance penalty
here is just that all these queues share a common lock
so stateful operations on one queue would imply a delay
on other queues.

For older kernels than 2.6.23 we can only stop all the
queues then and wake them up only if no other queue had
been stopped previously. This means for kernels older
than 2.6.23 there is a performance penalty and congestion
on one queue would imply propagating the same congestion
impact on all the other queues.

The select_queue callback was only added as of 2.6.27 via
commit eae792b7 so for kernels older than 2.6.23 and up
to 2.6.27 we must ensure we do the selection of the queue
once the core networking calls mac80211's dev_hard_start_xmit()
(ndo_start_xmit() callback on newer kernels).

This patch then consists of three parts:

  1) Addresses the lack of select_queue on older kernels than 2.6.27
  2) Extends the backport of net_device_ops for select_queue for kernels >= 2.6.27
  3) Backporting wake/stop queue for older kernels:
	- Handle with net_device_subqueue for >= 2.6.23
	- Treat each queue operation as an aggregate for all queues

Monitor interfaces have their own select_queue -- monitor interfaces
are used for injecting frames so they have their own respective queue
handling, but mac80211 just always sends management frames on VO
queue by using skb_set_queue_mapping(skb, 0) through ieee80211_tx_skb()

--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -265,6 +265,18 @@ __le16 ieee80211_ctstoself_duration(stru
 }
 EXPORT_SYMBOL(ieee80211_ctstoself_duration);
 
+#if (LINUX_VERSION_CODE < KERNEL_VERSION(2,6,23))
+static bool ieee80211_all_queues_started(struct ieee80211_hw *hw)
+{
+	unsigned int queue;
+
+	for (queue = 0; queue < hw->queues; queue++)
+		if (ieee80211_queue_stopped(hw, queue))
+			return false;
+	return true;
+}
+#endif
+
 static void __ieee80211_wake_queue(struct ieee80211_hw *hw, int queue,
 				   enum queue_stop_reason reason)
 {
@@ -287,7 +299,14 @@ static void __ieee80211_wake_queue(struc
 		list_for_each_entry_rcu(sdata, &local->interfaces, list) {
 			if (test_bit(SDATA_STATE_OFFCHANNEL, &sdata->state))
 				continue;
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,27))
 			netif_wake_subqueue(sdata->dev, queue);
+#elif (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,23))
+			netif_start_subqueue(sdata->dev, queue);
+#else
+			if (ieee80211_all_queues_started(hw))
+				netif_wake_queue(sdata->dev);
+#endif
 		}
 		rcu_read_unlock();
 	} else
@@ -327,7 +346,13 @@ static void __ieee80211_stop_queue(struc
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(sdata, &local->interfaces, list)
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,27))
+		netif_stop_subqueue(sdata->dev, queue);
+#elif (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,23))
 		netif_stop_subqueue(sdata->dev, queue);
+#else
+		netif_stop_queue(sdata->dev);
+#endif
 	rcu_read_unlock();
 }
 
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -1473,6 +1473,10 @@ void ieee80211_xmit(struct ieee80211_sub
 				return;
 			}
 
+#if (LINUX_VERSION_CODE < KERNEL_VERSION(2,6,27))
+	/* Older kernels do not have the select_queue callback */
+	skb_set_queue_mapping(skb, ieee80211_select_queue(sdata, skb));
+#endif
 	ieee80211_set_qos_hdr(sdata, skb);
 	ieee80211_tx(sdata, skb, false);
 	rcu_read_unlock();
@@ -2208,7 +2212,14 @@ void ieee80211_tx_pending(unsigned long
 
 		if (skb_queue_empty(&local->pending[i]))
 			list_for_each_entry_rcu(sdata, &local->interfaces, list)
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,27))
 				netif_wake_subqueue(sdata->dev, i);
+#elif (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,23))
+				netif_start_subqueue(sdata->dev, i);
+#else
+				if (ieee80211_all_queues_started(hw))
+					netif_wake_queue(sdata->dev);
+#endif
 	}
 	spin_unlock_irqrestore(&local->queue_stop_reason_lock, flags);