pipes: Reorder functions

Checkpoint and restore do use fdtype_ops
and file_desc_ops intensively so reorder
pipes functions, this allow us to make
them static and drop additional declarations.

No func changes.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>