perf test: Fix non-bash issue with stat bpf counters

$(( .. )) is a bash feature but the test's interpreter is !/bin/sh,
switch the code to use expr.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: bpf@vger.kernel.org
Link: http://lore.kernel.org/lkml/20210617184216.2075588-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: captain5050

tools/perf/tests/shell/stat_bpf_counters.sh

@@ -11,9 +11,9 @@ compare_number()
        second_num=$2
 
        # upper bound is first_num * 110%
-       upper=$(( $first_num + $first_num / 10 ))
+       upper=$(expr $first_num + $first_num / 10 )
        # lower bound is first_num * 90%
-       lower=$(( $first_num - $first_num / 10 ))
+       lower=$(expr $first_num - $first_num / 10 )
 
        if [ $second_num -gt $upper ] || [ $second_num -lt $lower ]; then
                echo "The difference between $first_num and $second_num are greater than 10%."