zdtm/tempfs: set mode for O_CREAT

man 2 open:
"""
mode specifies the permissions to use in case a new file is cre‐
ated.  This argument must be supplied when O_CREAT or O_TMPFILE
is specified in flags;
"""

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