perf tests attr: Fix sample_period setup

perf tests attr: Fix sample_period setup

The final period can differ from what user specifies on command line due
to the perf_event_max_sample_rate sysctl setup.

Thus we can't predixt the sample_period value any more.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20170703145030.12903-12-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>