c/members_backend: check domain while decr removed replicas

in calculate_unevenness_error we calculate current replicas per node by
domain. Previously we didn't check domain of replicas that are going to
be removed so we decremented allocated_replicas incorreclty.

Now we add check that removing replica has same domain as we are
counting now

src/v/cluster/members_backend.cc

@@ -178,7 +178,7 @@ double calculate_unevenness_error(
      * from the node after successful reallocation
      */
     for (const auto& r : update.partition_reallocations) {
-        if (r.allocation_units) {
+        if (get_allocation_domain(r.ntp) == domain && r.allocation_units) {
             for (const auto& to_remove : r.replicas_to_remove) {
                 auto it = node_replicas.find(to_remove);
                 if (it != node_replicas.end()) {