mgr/smb: Replace "clusters" with "shares" in error status

mgr/smb: Replace "clusters" with "shares" in error status

In the following error response the list of active shares is incorrectly
displayed with "clusters" field.
{
  "resource": {
    "resource_type": "ceph.smb.cluster",
    "cluster_id": "smbcluster",
    "intent": "removed"
  },
  "clusters": [
    "smbshare"
  ],
  "msg": "cluster in use by shares",
  "success": false
}

Replace "clusters" with "shares" to avoid confusion.

Signed-off-by: Anoop C S <anoopcs@cryptolab.net>