powerpc/selftests: Use timersub() for gettimeofday()

Use timersub() function to simplify the code.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220816105106.82666-1-ye.xingchen@zte.com.cn

tools/testing/selftests/powerpc/benchmarks/gettimeofday.c

@@ -12,7 +12,7 @@ static int test_gettimeofday(void)
 {
 	int i;
 
-	struct timeval tv_start, tv_end;
+	struct timeval tv_start, tv_end, tv_diff;
 
 	gettimeofday(&tv_start, NULL);
 
@@ -20,7 +20,9 @@ static int test_gettimeofday(void)
 		gettimeofday(&tv_end, NULL);
 	}
 
-	printf("time = %.6f\n", tv_end.tv_sec - tv_start.tv_sec + (tv_end.tv_usec - tv_start.tv_usec) * 1e-6);
+	timersub(&tv_start, &tv_end, &tv_diff);
+
+	printf("time = %.6f\n", tv_diff.tv_sec + (tv_diff.tv_usec) * 1e-6);
 
 	return 0;
 }