nvme: lightnvm: attach lightnvm sysfs to nvme block device

Previously, LBA read and write were not supported in the lightnvm
specification. Now that it supports it, lets use the traditional
NVMe gendisk, and attach the lightnvm sysfs geometry export.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>