s390/vdso: remove superfluous check

vdso_pages (aka vdso64_pages) is never 0, therefore remove the check.

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>