nvmet: Cleanup discovery execute handlers

Push the lid and cns check into their respective handlers and, while
we're at it, rename the functions to be consistent with other
discovery handlers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[split patch, update changelog]
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>