btrbk: filter for family=btrbk on archive

We don't support families other than "btrbk" yet: timeshift uses
subdirs, and there's too many places where the snapshot_dir is
supposed to be the directory where the snapshots reside in.

btrbk

@@ -5900,7 +5900,7 @@ MAIN:
     {
       next unless($vol->{node}{readonly});
       my $snapshot_name = $vol->{node}{BTRBK_BASENAME};
-      unless(defined($snapshot_name)) {
+      unless(defined($snapshot_name) && ($vol->{node}{BTRBK_FAMILY} eq "btrbk")) {
         WARN "Skipping subvolume (not a btrbk subvolume): $vol->{PRINT}";
         next;
       }