s390/vdso: simplify __arch_get_hw_counter()

Use the passed in vdso_data pointer instead of calculating it again.
This is also required as a prerequisite for vdso time namespaces: if a
process is part of a time namespace __arch_get_vdso_data() will return
a pointer to the time namespace data page instead of the vdso data
page, which is not what __arch_get_hw_counter() expects.

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>