mmap15: raise high address to 2^53

Address space on s390x can go atm. as high as 2^53,
so raise high address used in this test to this level.

Signed-off-by: Li Wang <wangli.ahau@gmail.com>
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Caspar Zhang <caspar@casparzhang.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>