cgroup: use perror on failed property write

The errno here is useful information for debugging, we should also print
it.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cgroup.c

@@ -1237,7 +1237,7 @@ static int restore_cgroup_prop(const CgroupPropEntry * cg_prop_entry_p,
 
 	len = strlen(cg_prop_entry_p->value);
 	if (write(fd, cg_prop_entry_p->value, len) != len) {
-		pr_err("Failed writing %s to %s\n", cg_prop_entry_p->value, path);
+		pr_perror("Failed writing %s to %s\n", cg_prop_entry_p->value, path);
 		goto out;
 	}