s390/cpumf: move counter set size calculation to common place

The function to calculate the size of counter sets is renamed from
cf_diag_ctrset_size() to cpum_cf_ctrset_size() and moved to the file
containing common functions for the CPU Measurement Counter Facility.
No functional change.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by : Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>