nvme-pci: iod's 'aborted' is a bool

It's only true or false, so make this a bool to reflect that and save
some space in nvme_iod.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>