zdtm: open an one unlinked files two times

A few test check restoring of file descriptors to a one file struct.
Here is better to check, that a few file struct can be linked to one unlinked file.

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