nvme: prepare for fault injection into admin commands

Currenlty fault injection support for nvme only enables to inject errors
into the commands submitted to I/O queues.

In preparation for fault injection into the admin commands, this makes
the helper functions independent of struct nvme_ns.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>