block: null_blk: Cleanup device creation and deletion

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2083917



commit b3a0a73e
Author: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Date:   Wed Apr 20 09:57:16 2022 +0900

    block: null_blk: Cleanup device creation and deletion

    Introduce the null_create_dev() and null_destroy_dev() helper functions
    to respectivel create nullb devices on modprobe and destroy them on
    rmmod. The null_destroy_dev() helper avoids duplicated code in the
    null_init() and null_exit() functions for deleting devices.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Link: https://lore.kernel.org/r/20220420005718.3780004-3-damien.lemoal@opensource.wdc.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>