bcache: improve error message in bch_cached_dev_run()

This patch adds more error message in bch_cached_dev_run() to indicate
the exact reason why an error value is returned. Please notice when
printing out the "is running already" message, pr_info() is used here,
because in this case also -EBUSY is returned, the bcache device can
continue to attach to the cache devince and run, so it won't be an
error level message in kernel message.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -926,13 +926,18 @@ int bch_cached_dev_run(struct cached_dev *dc)
 		NULL,
 	};
 
-	if (dc->io_disable)
+	if (dc->io_disable) {
+		pr_err("I/O disabled on cached dev %s",
+		       dc->backing_dev_name);
 		return -EIO;
+	}
 
 	if (atomic_xchg(&dc->running, 1)) {
 		kfree(env[1]);
 		kfree(env[2]);
 		kfree(buf);
+		pr_info("cached dev %s is running already",
+		       dc->backing_dev_name);
 		return -EBUSY;
 	}
 
@@ -961,7 +966,7 @@ int bch_cached_dev_run(struct cached_dev *dc)
 	if (sysfs_create_link(&d->kobj, &disk_to_dev(d->disk)->kobj, "dev") ||
 	    sysfs_create_link(&disk_to_dev(d->disk)->kobj,
 			      &d->kobj, "bcache")) {
-		pr_debug("error creating sysfs link");
+		pr_err("Couldn't create bcache dev <-> disk sysfs symlinks");
 		return -ENOMEM;
 	}