refperf: Work around 64-bit division

A 64-bit division was introduced in refperf, breaking compilation
on all 32-bit architectures:

kernel/rcu/refperf.o: in function `main_func':
refperf.c:(.text+0x57c): undefined reference to `__aeabi_uldivmod'

Fix this by using div_u64 to mark the expensive operation.

[ paulmck: Update primitive and format per Nathan Chancellor. ]
Fixes: bd5b16d6c88d ("refperf: Allow decimal nanoseconds")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Valdis Klētnieks <valdis.kletnieks@vt.edu>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refperf.c

@@ -478,7 +478,7 @@ static int main_func(void *arg)
 		if (torture_must_stop())
 			goto end;
 
-		result_avg[exp] = 1000 * process_durations(nreaders) / (nreaders * loops);
+		result_avg[exp] = div_u64(1000 * process_durations(nreaders), nreaders * loops);
 	}
 
 	// Print the average of all experiments
@@ -489,9 +489,13 @@ static int main_func(void *arg)
 	strcat(buf, "Runs\tTime(ns)\n");
 
 	for (exp = 0; exp < nruns; exp++) {
+		u64 avg;
+		u32 rem;
+
 		if (errexit)
 			break;
-		sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, result_avg[exp] / 1000, (int)(result_avg[exp] % 1000));
+		avg = div_u64_rem(result_avg[exp], 1000, &rem);
+		sprintf(buf1, "%d\t%llu.%03u\n", exp + 1, avg, rem);
 		strcat(buf, buf1);
 	}