drm/vmwgfx: Fix up user_dmabuf refcounting

If user space calls unreference on a user_dmabuf it will typically
kill the struct ttm_base_object member which is responsible for the
user-space visibility. However the dmabuf part may still be alive and
refcounted. In some situations, like for shared guest-backed surface
referencing/opening, the driver may try to reference the
struct ttm_base_object member again, causing an immediate kernel warning
and a later kernel NULL pointer dereference.

Fix this by always maintaining a reference on the struct
ttm_base_object member, in situations where it might subsequently be
referenced.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_shader.c

@@ -855,7 +855,7 @@ static int vmw_shader_define(struct drm_device *dev, struct drm_file *file_priv,
 
 	if (buffer_handle != SVGA3D_INVALID_ID) {
 		ret = vmw_user_dmabuf_lookup(tfile, buffer_handle,
-					     &buffer);
+					     &buffer, NULL);
 		if (unlikely(ret != 0)) {
 			DRM_ERROR("Could not find buffer for shader "
 				  "creation.\n");