nvme: clean up nvme_scan_work

Move the check for the supported CNS value into nvme_scan_ns_list, and
limit the life time of the identify controller allocation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>