NVMe: replace nvme_ns with nvme_dev for user admin

The function nvme_user_admin_command does not require a namespace to
proceed.  Replace with the nvme_dev structure so that it can be called
from contexts that do not have a namespace.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>