perf test: Fix test 21 for s390x

perf test: Fix test 21 for s390x

Test case 21 (Number of exit events of a simple workload) fails on
s390x. The reason is the invalid sample frequency supplied for this
test. On s390x the minimum sample frequency is much higher (see output
of /proc/service_levels).

Supply a save sample frequency value for s390x to fix this.  The value
will be adjusted by the s390x CPUMF frequency convertion function to a
value well below the sysctl kernel.perf_event_max_sample_rate value.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
LPU-Reference: 20171123114611.93397-1-tmricht@linux.vnet.ibm.com
Link: https://lkml.kernel.org/n/tip-1ynblyhi1n81idpido59nt1y@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>