dma-buf: might_sleep() in dma_buf_unmap_attachment()

We never really clarified if unmap could be done in atomic context.
But since mapping might require sleeping, this implies mutex in use
to synchronize mapping/unmapping, so unmap could sleep as well.  Add
a might_sleep() to clarify this.
Signed-off-by: Rob Clark <rob@ti.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

drivers/base/dma-buf.c

@@ -298,6 +298,8 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
 				struct sg_table *sg_table,
 				enum dma_data_direction direction)
 {
+	might_sleep();
+
 	if (WARN_ON(!attach || !attach->dmabuf || !sg_table))
 		return;