ublk: rewrite ublk_ctrl_get_queue_affinity to not rely on hctx->cpumask

ublk: rewrite ublk_ctrl_get_queue_affinity to not rely on hctx->cpumask

Looking at the hctxs and cpumap is not safe without at very last a RCU
reference.  It also requires the queue to be set up before starting the
device, which leads to rather awkward life time rules.

Instead rewrite ublk_ctrl_get_queue_affinity to just build the cpumask
directly from the mq_map in the tag set, similar to hctx->cpumask is
built.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>