nvme: don't allow unprivileged passthrough on partitions

Passthrough commands can always access the entire device, and thus
submitting them on partitions is an privelege escalation.

In hindsight we should have never allowed any passthrough commands on
partitions, but it's probably too late to change that decision now.

Fixes: e4fbcf32

 ("nvme: identify-namespace without CAP_SYS_ADMIN")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>