ring-buffer: check for divide by zero in ring-buffer-benchmark

Although we check if "missed" is not zero, we divide by hit + missed,
and the addition can possible overflow and become a divide by zero.

This patch checks for this case, and will report it when it happens
then modify "hit" to make the calculation be non zero.

[ Impact: prevent possible divide by zero in ring-buffer-benchmark ]
Reported-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer_benchmark.c

@@ -294,6 +294,12 @@ static void ring_buffer_producer(void)
 
 		pr_info("Total iterations per millisec: %ld\n", hit + missed);
 
+		/* it is possible that hit + missed will overflow and be zero */
+		if (!(hit + missed)) {
+			pr_info("hit + missed overflowed and totalled zero!\n");
+			hit--; /* make it non zero */
+		}
+
 		/* Caculate the average time in nanosecs */
 		avg = NSEC_PER_MSEC / (hit + missed);
 		pr_info("%ld ns per entry\n", avg);