arch: setup PF_IO_WORKER threads like PF_KTHREAD

arch: setup PF_IO_WORKER threads like PF_KTHREAD

[ Upstream commit 4727dc20e0422211a0e0c72b1ace4ed6096df8a6 ]

PF_IO_WORKER are kernel threads too, but they aren't PF_KTHREAD in the
sense that we don't assign ->set_child_tid with our own structure. Just
ensure that every arch sets up the PF_IO_WORKER threads like kthreads
in the arch implementation of copy_thread().

Signed-off-by: Jens Axboe <axboe@kernel.dk>