s390/cpumf: remove call to perf_event_update_userpage

The function cpumf_pmu_add and cpumf_pmu_del call function
perf_event_update_userpage(). This calls is obsolete, the calls add and
delete a counter event. Counter events do not sample data and the
event->rb member to access the sampling ring buffer is always NULL.
The function perf_event_update_userpage() simply returns in this case.

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>