block: null_blk: Cleanup messages

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



commit db060f54
Author: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Date:   Wed Apr 20 09:57:17 2022 +0900

    block: null_blk: Cleanup messages

    Use the pr_fmt() macro to prefix all null_blk pr_xxx() messages with
    "null_blk:" to clarify which module is printing the messages. Also add
    a pr_info() message in null_add_dev() to print the name of a newly
    created disk.

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/20220420005718.3780004-4-damien.lemoal@opensource.wdc.com


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

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