mount: don't close a namespace descriptor two times

CID null (#1 of 1): Double close (USE_AFTER_FREE)
10. double_close: Calling "restore_ns(int, struct ns_desc *)" closes
handle "ns_old" which has already been closed.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>