dmaengine: fix NULL pointer in channel unregistration function

__dma_async_device_channel_register() can fail. In case of failure,
chan->local is freed (with free_percpu()), and chan->local is nullified.
When dma_async_device_unregister() is called (because of managed API or
intentionally by DMA controller driver), channels are unconditionally
unregistered, leading to this NULL pointer:
[    1.318693] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000d0
[...]
[    1.484499] Call trace:
[    1.486930]  device_del+0x40/0x394
[    1.490314]  device_unregister+0x20/0x7c
[    1.494220]  __dma_async_device_channel_unregister+0x68/0xc0

Look at dma_async_device_register() function error path, channel device
unregistration is done only if chan->local is not NULL.

Then add the same condition at the beginning of
__dma_async_device_channel_unregister() function, to avoid NULL pointer
issue whatever the API used to reach this function.

Fixes: d2fb0a04 ("dmaengine: break out channel registration")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20231213160452.2598073-1-amelie.delaunay@foss.st.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dmaengine.c

@@ -1103,6 +1103,9 @@ EXPORT_SYMBOL_GPL(dma_async_device_channel_register);
 static void __dma_async_device_channel_unregister(struct dma_device *device,
 						  struct dma_chan *chan)
 {
+	if (chan->local == NULL)
+		return;
+
 	WARN_ONCE(!device->device_release && chan->client_count,
 		  "%s called while %d clients hold a reference\n",
 		  __func__, chan->client_count);