nfsd: simplify process_lock

Similarly, this STALE_CLIENTID check is already handled by:

nfs4_preprocess_confirmed_seqid_op()->
        nfs4_preprocess_seqid_op()->
                nfsd4_lookup_stateid()->
                        set_client()->
                                STALE_CLIENTID()

(This may cause it to return a different error in some cases where
there are multiple things wrong; pynfs test SEQ10 regressed on this
commit because of that, but I think that's the test's fault, and I've
fixed it separately.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>