Description of problem:
As title, upgrade failed and msg show cluster-backup fails

$oc get pods -n openshift-etcd
NAME                                                                  READY   STATUS      RESTARTS   AGE
cluster-backup                                                        0/1     Error       0          10s

$oc logs -f cluster-backup
Defaulted container "cluster-backup" out of: cluster-backup, verify-storage (init)
+ '[' -n /etc/kubernetes/cluster-backup ']'
+ echo 'removing all backups in /etc/kubernetes/cluster-backup'
+ rm -rf /etc/kubernetes/cluster-backup
removing all backups in /etc/kubernetes/cluster-backup
rm: cannot remove '/etc/kubernetes/cluster-backup': Device or resource busy

How reproducible:
Always

Steps to Reproduce:
1. upgrade from 4.14.7 to 4.15.0-rc.0
2. upgrade failed:

 message: |-
      Preconditions failed for payload loaded version="4.15.0-rc.0" image="quay.io/openshift-release-dev/ocp-release@sha256:a3b96bf3aef71d4062cae3961fcba3bb3019b52ab2da60b2997d4842f9cdcf07": Multiple precondition checks failed:
      * Precondition "ClusterVersionUpgradeable" failed because of "PoolUpdating": Cluster operator machine-config should not be upgraded between minor versions: One or more machine config pools are updating, please see `oc get mcp` for further details
      * Precondition "EtcdRecentBackup" failed because of "UpgradeBackupFailed": RecentBackup: pod failed within retry duration: delete skipped
    reason: PreconditionChecks
    status: "False"
    type: ReleaseAccepted

Actual results:
As title
Expected results
Upgrade succ