btrfs-progs: make filesystem show by label work
    
    with design revamp around filesystem show the fsid filter
    by label wasn't planned. but apparently that seemed to be
    necessary. this patch will fix it.
    
    Signed-off-by: Anand Jain <anand.jain@oracle.com>
    Signed-off-by: David Sterba <dsterba@suse.cz>