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>

fs/nfsd/cache.h

@@ -69,7 +69,7 @@ enum {
 
 int	nfsd_reply_cache_init(void);
 void	nfsd_reply_cache_shutdown(void);
-int	nfsd_cache_lookup(struct svc_rqst *, int);
+int	nfsd_cache_lookup(struct svc_rqst *);
 void	nfsd_cache_update(struct svc_rqst *, int, __be32 *);
 
 #ifdef CONFIG_NFSD_V4