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.mountain


Signed-off-by: Jakub Kicinski <kuba@kernel.org>