perf test: raise limit to 20 percent for perf_stat_--bpf-counters_test

perf test: raise limit to 20 percent for perf_stat_--bpf-counters_test

This test case often fails on s390 (about 2 out of 10) because the
10% percent limit on the difference between --bpf-counters event counting
and s390 hardware counting is more than 10% in all failure cases.
Raise the limit to 20% on s390 and the test case succeeds.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: gor@linux.ibm.com
Cc: hca@linux.ibm.com
Cc: sumanthk@linux.ibm.com
Cc: svens@linux.ibm.com
Link: https://lore.kernel.org/r/20240108084009.3959211-1-tmricht@linux.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>