nvmet: add debugfs support

Add a debugfs hierarchy to display the configured subsystems
and the controllers attached to the subsystems.

Suggested-by: Redouane BOUFENGHOUR <redouane.boufenghour@shadow.tech>
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>