block: don't allow to switch elevator if updating nr_hw_queues is in-progress

block: don't allow to switch elevator if updating nr_hw_queues is in-progress

Elevator switch code is another `nr_hw_queue` reader in non-fast-IO code
path, so it can't be done if updating `nr_hw_queues` is in-progress.

Take same approach with not allowing add/del disk when updating
nr_hw_queues is in-progress, by grabbing read lock of
set->update_nr_hwq_sema.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/linux-block/aAWv3NPtNIKKvJZc@fedora/
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Closes: https://lore.kernel.org/linux-block/mz4t4tlwiqjijw3zvqnjb7ovvvaegkqganegmmlc567tt5xj67@xal5ro544cnc/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-10-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>