btrfs: Add btrfs: prefix to kernel log output

The kernel log entries for device label %s and device fsid %pU
are missing the btrfs: prefix. Add those here.
Signed-off-by: NFrank Holton <fholton@gmail.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/volumes.c

@@ -912,9 +912,9 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 	if (disk_super->label[0]) {
 		if (disk_super->label[BTRFS_LABEL_SIZE - 1])
 			disk_super->label[BTRFS_LABEL_SIZE - 1] = '\0';
-		printk(KERN_INFO "device label %s ", disk_super->label);
+		printk(KERN_INFO "btrfs: device label %s ", disk_super->label);
 	} else {
-		printk(KERN_INFO "device fsid %pU ", disk_super->fsid);
+		printk(KERN_INFO "btrfs: device fsid %pU ", disk_super->fsid);
 	}
 
 	printk(KERN_CONT "devid %llu transid %llu %s\n", devid, transid, path);