nfsd: turn on reply cache for NFSv4

It's sort of ridiculous that we've never had a working reply cache for
NFSv4.

On the other hand, we may still not: our current reply cache is likely
not very good, especially in the TCP case (which is the only case that
matters for v4).  What we really need here is some serious testing.

Anyway, here's a start.

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