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: NAnil Madhavapeddy <anil@recoil.org>
Signed-off-by: NDaniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/gntalloc.c

@@ -191,6 +191,8 @@ static void __del_gref(struct gntalloc_gref *gref)
 
 		if (!gnttab_end_foreign_access_ref(gref->gref_id, 0))
 			return;
+
+		gnttab_free_grant_reference(gref->gref_id);
 	}
 
 	gref_size--;