zdtm/maps007: don't use signed values in calculations

maps007 segfaults on i386, because "size" is calculated bigger than allowed.

This occurs when the result of lrand48() * PAGE_SIZE is negative.
In this case the % operation returns a negative value too, what is unexpected.

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