rtlwifi: Preallocate USB read buffers and eliminate kalloc in read routine

The current version of rtlwifi for USB operations uses kmalloc to
acquire a 32-bit buffer for each read of the device. When
_usb_read_sync() is called with the rcu_lock held, the result is
a "sleeping function called from invalid context" BUG. This is
reported for two cases in https://bugzilla.kernel.org/show_bug.cgi?id=42775.
The first case has the lock originating from within rtlwifi and could
be fixed by rearranging the locking; however, the second originates from
within mac80211. The kmalloc() call is removed from _usb_read_sync()
by creating a ring buffer pointer in the private area and
allocating the buffer data in the probe routine.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> [This version good for 3.3+ - different patch for 3.2 - 2.6.39]
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rtlwifi/wifi.h

@@ -67,7 +67,7 @@
 #define QOS_QUEUE_NUM				4
 #define RTL_MAC80211_NUM_QUEUE			5
 #define REALTEK_USB_VENQT_MAX_BUF_SIZE		254
-
+#define RTL_USB_MAX_RX_COUNT			100
 #define QBSS_LOAD_SIZE				5
 #define MAX_WMMELE_LENGTH			64
 
@@ -1629,6 +1629,10 @@ struct rtl_priv {
 	   interface or hardware */
 	unsigned long status;
 
+	/* data buffer pointer for USB reads */
+	__le32 *usb_data;
+	int usb_data_index;
+
 	/*This must be the last item so
 	   that it points to the data allocated
 	   beyond  this structure like: