For the benefit of anyone coming into this thread with an interest in NFS + Kubernetes + LINSTOR, since version 2.10.0, the LINSTOR Operator has RWX persistent volumes support.
Release announcement: LINSTOR Operator 2.10.0
This LINBIT blog article, Creating ReadWriteMany Persistent Volumes in Kubernetes by using LINSTOR, explores the feature in detail and shows how to configure RWX access for PersistentVolumes.