powerpc/kernel: Rename copy_thread() '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.

Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>