ptp: drop redundant kasprintf() to create worker name

Building with -Wformat-nonliteral, gcc complains

drivers/ptp/ptp_clock.c: In function ‘ptp_clock_register’:
drivers/ptp/ptp_clock.c:239:26: warning: format not a string literal and no format arguments [-Wformat-nonliteral]
            worker_name : info->name);

kthread_create_worker takes fmt+varargs to set the name of the
worker, and that happens with a vsnprintf() to a stack buffer (that is
then copied into task_comm). So there's no reason not to just pass
"ptp%d", ptp->index to kthread_create_worker() and avoid the
intermediate worker_name variable.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>