namespaces: close fd on the source netns in restore_ns

It's opened in switch_ns.

CID 996194 (#3 of 5): Resource leak (RESOURCE_LEAK)
11. leaked_handle: Handle variable rst going out of scope leaks the handle.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

namespaces.c

@@ -58,6 +58,8 @@ int restore_ns(int rst, struct ns_desc *nd)
 	if (ret < 0)
 		pr_perror("Can't restore ns back");
 
+	close(rst);
+
 	return ret;
 }