nvme: merge nvme_ns_ioctl into nvme_ioctl

[ Upstream commit 90ec611adcf20b96d0c2b7166497d53e4301a57f ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>