Reason for the lock failure:
Because of the sudden ungraceful shutdown of the database, file locks on data files were not released by the NFS server (ZFS SA in this case). NFS server held on to the file locks even after the NFS client (DB server node in this example) was restarted. Due to this, Oracle RDBMS is not able to lock those data files residing on NFS server (ZFS SA). As a result, database instance was failed to start up in exclusive mode.
Manually clear the NFS locks as outlined below.
On NFS Client (database server node):
Shutdown the mounted database
Unmount remote (NFS) filesystems
# clear_locks -s <nfs_server_host>
# clear_locks -s zfs_nfs_server