nvmet: refactor configfs transport type handling

Have a common table of mappings from numerical transport ids to names, and
zero the transport specific area in common code in nvmet_addr_trtype_store.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>