NFSv4.1 cleanup DS stateid error handling

The error handler nfs4_state parameter is never NULL in the pNFS case as
the open_context must carry an nfs_state.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>