restore: call close_old_fds() after mounting /proc

A process can be restored in a new pidns. close_old_fds() opens
the /proc/PID directory. Without this patch we can see errors like this:
(00.333915)      1: Error (util.c:102): Unable to close fd 6: Bad file descriptor

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