nbd: add missing definition of pr_fmt

nbd: add missing definition of pr_fmt

commit 1243172d5894 ("nbd: use pr_err to output error message") tries
to define pr_fmt and use short pr_err() to output error message,
however, the definition is missed.

This patch also remove existing "nbd:" inside pr_err().

Fixes: 1243172d5894 ("nbd: use pr_err to output error message")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20220723082427.3890655-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>