nfsd: prepare nfsd4_cb_sequence_done() for error handling rework

nfsd: prepare nfsd4_cb_sequence_done() for error handling rework

There is only one case where we want to proceed with processing the rest
of the CB_COMPOUND, and that's when the cb_seq_status is 0. Make the
default return value be false, and only set it to true in that case.

Rename the "need_restart" label to "requeue", to better indicate that
it's being requeued to the workqueue.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>