mips: 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>
Cc: linux-kernel@vger.kernel.org
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Alex Smith <alex@alex-smith.me.uk>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Eunbong Song <eunb.song@samsung.com>
Cc: linux-mips@linux-mips.org (open list:MIPS)
Patchwork: https://patchwork.linux-mips.org/patch/9546/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>