nvme: expose hostnqn via sysfs for fabrics controllers

We allow userspace to connect with a custom hostnqn which is useful for
certain use-cases. However there is no way to tell what is the hostnqn
used to connect to a given controller.

Expose this so userspace can correlate controllers based on hostnqn.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>