nvme_core_init does:
    1) register_blkdev
    2) __register_chrdev
    3) class_create

nvme_core_exit should do cleanup in the reverse order.
Signed-off-by: Wang Sheng-Hui <shhuiw@foxmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>