So we keep it and dont close inside close_old_fds()
helper but pass into veth creation so the kernel
can fetch the net namespace of the veth peer.

v2 (by avagin@):
 - don't forget to close opened descriptor
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>