nvme: export nvme_find_get_ns() and nvme_put_ns()

nvme_find_get_ns() and nvme_put_ns() are required by the target passthru
code and are exported under the NVME_TARGET_PASSTHRU namespace.

Based-on-a-patch-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>