xen/dmabuf: switch gntdev-dmabuf to use INVALID_GRANT_REF

Instead of using a private macro for an invalid grant reference use
the common one.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/gntdev-dmabuf.c

@@ -24,15 +24,6 @@
 
 MODULE_IMPORT_NS(DMA_BUF);
 
-#ifndef GRANT_INVALID_REF
-/*
- * Note on usage of grant reference 0 as invalid grant reference:
- * grant reference 0 is valid, but never exposed to a driver,
- * because of the fact it is already in use/reserved by the PV console.
- */
-#define GRANT_INVALID_REF	0
-#endif
-
 struct gntdev_dmabuf {
 	struct gntdev_dmabuf_priv *priv;
 	struct dma_buf *dmabuf;
@@ -532,7 +523,7 @@ static void dmabuf_imp_end_foreign_access(u32 *refs, int count)
 	int i;
 
 	for (i = 0; i < count; i++)
-		if (refs[i] != GRANT_INVALID_REF)
+		if (refs[i] != INVALID_GRANT_REF)
 			gnttab_end_foreign_access(refs[i], 0UL);
 }
 
@@ -567,7 +558,7 @@ static struct gntdev_dmabuf *dmabuf_imp_alloc_storage(int count)
 	gntdev_dmabuf->nr_pages = count;
 
 	for (i = 0; i < count; i++)
-		gntdev_dmabuf->u.imp.refs[i] = GRANT_INVALID_REF;
+		gntdev_dmabuf->u.imp.refs[i] = INVALID_GRANT_REF;
 
 	return gntdev_dmabuf;