nvme: cleanup how disk->disk_name is assigned

They way how assigning the disk name and commenting on why it is done
is split over core.c and multipath.c seems to be rather confusing.

Now that ns_head->disk always exists we can do all the work in core.c
and have a single big comment explaining the issues.

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