null_blk: format pr_* logs with pr_fmt

Instead of writing "null_blk: " at the beginning of each
pr_err/info/warn log message, format messages using pr_fmt() macro.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>