NVMe: nvme_core_exit() should do cleanup in the reverse order as nvme_core_init does

NVMe: nvme_core_exit() should do cleanup in the reverse order as nvme_core_init does

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>
(cherry picked from commit 23bd63ceea30878758c303baaf9f8e28f299c578)

Orabug: 25130845

Signed-off-by: Ashok Vairavan <ashok.vairavan@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>