ptp: prevent string overflow

ptp: prevent string overflow

The ida_alloc_max() function can return up to INT_MAX so this buffer is
not large enough.  Also use snprintf() for extra safety.

Fixes: 403376ddb422 ("ptp: add debugfs interface to see applied channel masks")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/d4b1a995-a0cb-4125-aa1d-5fd5044aba1d@moroto.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>