nfs41: get_clid_cred for EXCHANGE_ID

Unlike SETCLIENTID, EXCHANGE_ID requires a machine credential. Do not search
for credentials other than the machine credential.

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>