parisc: copy_thread(): rename 'arg' argument to 'kthread_arg'

The 'arg' argument to copy_thread() is only ever used when forking a new
kernel thread. Hence, rename it to 'kthread_arg' for clarity (and
consistency with do_fork() and other arch-specific implementations of
copy_thread()).

Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>