nfsd4: skip encoder in trivial error cases

Most encoders do nothing in the error case.  But they can still screw
things up in that case: most errors happen very early in rpc processing,
possibly before argument fields are filled in and bounds-tested, so
encoders that do anything other than immediately bail on error can
easily crash in odd error cases.

So just handle errors centrally most of the time to remove the chance of
error.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>