Description of problem:
MetalLB on dual stack cluster can assign IP address to LoadBalancer service type from either address pool of IPv4 addresses or IPv6 addresses. If an addresspool has both types of addresses, the service gets stuck at 'Pending address' state when one type of IP address is used up.
 

Version-Release number of selected component (if applicable):
4.9

How reproducible:
Always

Steps to Reproduce:
1. Install MetalLB operator on dual stack cluster on baremetal
2. Configure address pool with range of IPv6 and IPv4 addresses.
 example 

    - fd2e:6f44:5dd8::6d - fd2e:6f44:5dd8::6d
    - fd2e:6f44:5dd8::91 - fd2e:6f44:5dd8::91
    - 192.168.123.126 - 192.168.123.126
    - 192.168.123.146 - 192.168.123.146

3. Create a service with following YAML that can be assigned either of the address.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: hello-world
  labels:
    app: hello-world
spec:
  selector:
    matchLabels:
      app: hello-world
  replicas: 10
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
      - name: hello-world
        image: gcr.io/google-samples/node-hello:1.0
        imagePullPolicy: Always
        ports:
        - containerPort: 8080
          protocol: TCP
 
---
apiVersion: v1
kind: Service
metadata:
  name: hello-world
spec:
  ipFamilyPolicy: RequireDualStack
  ipFamilies:
  - IPv6
  - IPv4
  selector:
    app: hello-world
  ports:
  - port: 80
    targetPort: 8080
    protocol: TCP
  type: LoadBalancer

Actual results:

Once all the addresses of first type are used up second type cannot be assigned.

Expected results:
Both types should be assigned to the service from the address pool.

Additional info:

Work around:- Create two different address pool - IPv4 address pool and IPv6 address pool so that service can be created with both type of addresses.

LoadBalancer service can have either IPv4 or IPv6 address not both.
https://github.com/kubernetes/enhancements/pull/1992#issuecomment-825905432