nvme: merge nvme_ns_ioctl into nvme_ioctl

Merge the two functions to make future changes a little easier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>