dma-debug: Do not add notifier when dma debugging is disabled.

If CONFIG_HAVE_DMA_API_DEBUG is defined and "dma_debug=off" is
specified on the kernel command line, when you detach a driver from a
device you can cause the following NULL pointer dereference:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<c0580d35>] dma_debug_device_change+0x5d/0x117

The problem is that the dma_debug_device_change notifier function is
added to the bus notifier chain even though the dma_entry_hash array
was never initialized.  If dma debugging is disabled, this patch both
prevents dma_debug_device_change notifiers from being added to the
chain, and additionally ensures that the dma_debug_device_change
notifier function is a no-op.

Cc: stable@kernel.org
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

lib/dma-debug.c

@@ -676,6 +676,8 @@ static int dma_debug_device_change(struct notifier_block *nb,
 	struct device *dev = data;
 	int count;
 
+	if (global_disable)
+		return;
 
 	switch (action) {
 	case BUS_NOTIFY_UNBOUND_DRIVER:
@@ -697,6 +699,9 @@ void dma_debug_add_bus(struct bus_type *bus)
 {
 	struct notifier_block *nb;
 
+	if (global_disable)
+		return;
+
 	nb = kzalloc(sizeof(struct notifier_block), GFP_KERNEL);
 	if (nb == NULL) {
 		pr_err("dma_debug_add_bus: out of memory\n");