s390/cpum_cf_diag: use get_tod_clock_fast()

Use get_tod_clock_fast() instead of store_tod_clock(), since
store_tod_clock() can be very slow.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>