io-wq: re-set NUMA node affinities if CPUs come online

We correctly set io-wq NUMA node affinities when the io-wq context is
setup, but if an entire node CPU set is offlined and then brought back
online, the per node affinities are broken. Ensure that we set them
again whenever a CPU comes online. This ensures that we always track
the right node affinity. The usual cpuhp notifiers are used to drive it.

Reported-by: Zhang Qiang <qiang.zhang@windriver.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>