s390/vdso: fix vdso data page definition

The vdso data page actually contains an array. Fix that.
This doesn't fix a real bug, just reflects reality.

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>