s390/vdso: simplify vdso size calculation

The vdso is (and must) be page aligned and its size must also be
a multiple of PAGE_SIZE. Therefore no need to round upwards.

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>