staging: android: ion: Check return value of ion_buffer_kmap_get

GCC warns that vaddr is set but unused. Check the return value of
ion_buffer_kmap_get to make vaddr useful and make sure everything
is properly configured before beginning a DMA.
Suggested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion.c

@@ -315,6 +315,7 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
 	struct ion_buffer *buffer = dmabuf->priv;
 	void *vaddr;
 	struct ion_dma_buf_attachment *a;
+	int ret = 0;
 
 	/*
 	 * TODO: Move this elsewhere because we don't always need a vaddr
@@ -322,6 +323,10 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
 	if (buffer->heap->ops->map_kernel) {
 		mutex_lock(&buffer->lock);
 		vaddr = ion_buffer_kmap_get(buffer);
+		if (IS_ERR(vaddr)) {
+			ret = PTR_ERR(vaddr);
+			goto unlock;
+		}
 		mutex_unlock(&buffer->lock);
 	}
 
@@ -330,9 +335,10 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
 		dma_sync_sg_for_cpu(a->dev, a->table->sgl, a->table->nents,
 				    direction);
 	}
-	mutex_unlock(&buffer->lock);
 
-	return 0;
+unlock:
+	mutex_unlock(&buffer->lock);
+	return ret;
 }
 
 static int ion_dma_buf_end_cpu_access(struct dma_buf *dmabuf,