xen/gntalloc: release grant references on page free

gnttab_end_foreign_access_ref does not return the grant reference it is
passed to the free list; gnttab_free_grant_reference needs to be
explicitly called. While gnttab_end_foreign_access provides a wrapper
for this, it is unsuitable because it does not return errors.

Reported-by: Anil Madhavapeddy <anil@recoil.org>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/drivers/xen/gntalloc.c b/drivers/xen/gntalloc.c
index c95181f..f330a4b 100644
--- a/drivers/xen/gntalloc.c
+++ b/drivers/xen/gntalloc.c
@@ -191,6 +191,8 @@
 
 		if (!gnttab_end_foreign_access_ref(gref->gref_id, 0))
 			return;
+
+		gnttab_free_grant_reference(gref->gref_id);
 	}
 
 	gref_size--;