nvme: allow unprivileged passthrough of Identify Controller

Add unprivileged passthrough of the I/O Command Set Independent and I/O
Command Set Specific Identify Controller sub-command.

This will allow access to attributes (e.g. MDTS and WZSL) that are needed
to effectively form passthrough I/O to the /dev/ng* character devices.

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