nvmet: add 'host_traddr' callback for debugfs

We want to display the transport address of the connected host
in debugfs, but this is a property of the transport.
So add a callback 'host_traddr' to allow the transport drivers
to fill in the data.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>