s390/vdso: move data page before code pages

For consistency with x86 and arm64 move the data page before code
pages. Similar to commit 601255ae ("arm64: vdso: move data page
before code pages").

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>