nvme: open code nvme_{get,put}_ns_from_disk in nvme_ns_head_ioctl

nvme_ns_head_ioctl is always used on multipath nodes, no need to
deal with the de-multiplexers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>