nvmet: add generic type-name mapping

This patch adds a new type to name mapping generic structure. It
replaces nvmet_transport_name with new generic mapping structure
nvmet_transport.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>