nbd: move setting the cache control flags to __nbd_set_size

nbd: move setting the cache control flags to __nbd_set_size

Move setting the cache control flags in nbd in preparation for moving
these flags into the queue_limits structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20240617060532.127975-11-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>