block: move the synchronous flag to queue_limits

Move the synchronous flag into the queue_limits feature field so that it
can be set atomically with the queue frozen.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20240617060532.127975-19-hch@lst.de


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