nvmet: implement crto property

This property is required for nvme 2.1. The target only supports ready
with media, so this is just the same value as CAP.TO.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Matias Bjørling <matias.bjorling@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>