ptp: fix integer overflow in max_vclocks_store

On 32bit systems, the "4 * max" multiply can overflow.  Use kcalloc()
to do the allocation to prevent this.

Fixes: 44c494c8 ("ptp: track available ptp vclocks information")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Heng Qi <hengqi@linux.alibaba.com>
Link: https://lore.kernel.org/r/ee8110ed-6619-4bd7-9024-28c1f2ac24f4@moroto.mountainSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/ptp/ptp_sysfs.c

@@ -296,8 +296,7 @@ static ssize_t max_vclocks_store(struct device *dev,
 	if (max < ptp->n_vclocks)
 		goto out;
 
-	size = sizeof(int) * max;
-	vclock_index = kzalloc(size, GFP_KERNEL);
+	vclock_index = kcalloc(max, sizeof(int), GFP_KERNEL);
 	if (!vclock_index) {
 		err = -ENOMEM;
 		goto out;