blk-mq: clear hctx->dispatch_from when mappings change

When the number of hardware queues is changed, the drivers will call
blk_mq_update_nr_hw_queues() to remap hardware queues. This changes
the ctx mappings, but the current code doesn't clear the
->dispatch_from hint. This can result in dispatch_from pointing to
a ctx that isn't mapped to the hctx anymore.

Fixes: b347689f ("blk-mq-sched: improve dispatching from sw queue")
Signed-off-by: huhai <huhai@kylinos.cn>
Reviewed-by: Ming Lei <ming.lei@redhat.com>

Moved the placement of the clearing to where we clear other items
pertaining to the existing mapping, added Fixes line, and reworded
the commit message.

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