btrfs: remove stub device info from messages when we have no fs_info

Without a NULL fs_info the helpers will print something like

	BTRFS error (device <unknown>): ...

This can happen in contexts where fs_info is not available at all or
it's potentially unsafe due to object lifetime. The <unknown> stub does
not bring much information and with the prefix makes the message
unnecessarily longer.

Remove it for the NULL fs_info case.

	BTRFS error: ...

Callers can add the device information to the message itself if needed.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -240,9 +240,13 @@ void __cold btrfs_printk(const struct btrfs_fs_info *fs_info, const char *fmt, .
 	vaf.fmt = fmt;
 	vaf.va = &args;
 
-	if (__ratelimit(ratelimit))
-		printk("%sBTRFS %s (device %s): %pV\n", lvl, type,
-			fs_info ? fs_info->sb->s_id : "<unknown>", &vaf);
+	if (__ratelimit(ratelimit)) {
+		if (fs_info)
+			printk("%sBTRFS %s (device %s): %pV\n", lvl, type,
+				fs_info->sb->s_id, &vaf);
+		else
+			printk("%sBTRFS %s: %pV\n", lvl, type, &vaf);
+	}
 
 	va_end(args);
 }