nvme: add nvme_get_ns helper

Add a helper to avoid opencoding ns->kref increment.
Decrement is already done via nvme_put_ns helper.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>