nbd: add missing definition of pr_fmt

commit 1243172d ("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: 1243172d ("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.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -11,6 +11,8 @@
  * (part of code stolen from loop.c)
  */
 
+#define pr_fmt(fmt) "nbd: " fmt
+
 #include <linux/major.h>
 
 #include <linux/blkdev.h>
@@ -1950,7 +1952,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 			     test_bit(NBD_DISCONNECT_REQUESTED, &nbd->flags)) ||
 			    !refcount_inc_not_zero(&nbd->refs)) {
 				mutex_unlock(&nbd_index_mutex);
-				pr_err("nbd: device at index %d is going down\n",
+				pr_err("device at index %d is going down\n",
 					index);
 				return -EINVAL;
 			}
@@ -1961,7 +1963,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 	if (!nbd) {
 		nbd = nbd_dev_add(index, 2);
 		if (IS_ERR(nbd)) {
-			pr_err("nbd: failed to add new device\n");
+			pr_err("failed to add new device\n");
 			return PTR_ERR(nbd);
 		}
 	}