nfs41: add a get_clid_cred function to nfs4_state_recovery_ops

EXCHANGE_ID has different credential requirements than SETCLIENTID.
Prepare for a separate credential function.

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