dma-buf: begin/end_cpu might lock the dma_resv lock

At least amdgpu and i915 do, so lets just document this as the rule.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org

drivers/dma-buf/dma-buf.c

@@ -1118,6 +1118,8 @@ int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
 	if (WARN_ON(!dmabuf))
 		return -EINVAL;
 
+	might_lock(&dma_buf->resv.lock);
+
 	if (dmabuf->ops->begin_cpu_access)
 		ret = dmabuf->ops->begin_cpu_access(dmabuf, direction);
 
@@ -1151,6 +1153,8 @@ int dma_buf_end_cpu_access(struct dma_buf *dmabuf,
 
 	WARN_ON(!dmabuf);
 
+	might_lock(&dma_buf->resv.lock);
+
 	if (dmabuf->ops->end_cpu_access)
 		ret = dmabuf->ops->end_cpu_access(dmabuf, direction);