NFSD: Skip extra computation for RC_NOCACHE case

Force the compiler to skip unneeded initialization for cases that
don't need those values. For example, NFSv4 COMPOUND operations are
RC_NOCACHE.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfscache.c

@@ -421,10 +421,10 @@ out:
  */
 int nfsd_cache_lookup(struct svc_rqst *rqstp)
 {
-	struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
+	struct nfsd_net		*nn;
 	struct svc_cacherep	*rp, *found;
 	__wsum			csum;
-	struct nfsd_drc_bucket	*b = nfsd_cache_bucket_find(rqstp->rq_xid, nn);
+	struct nfsd_drc_bucket	*b;
 	int type = rqstp->rq_cachetype;
 	int rtn = RC_DOIT;
 
@@ -440,10 +440,12 @@ int nfsd_cache_lookup(struct svc_rqst *rqstp)
 	 * Since the common case is a cache miss followed by an insert,
 	 * preallocate an entry.
 	 */
+	nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
 	rp = nfsd_reply_cache_alloc(rqstp, csum, nn);
 	if (!rp)
 		goto out;
 
+	b = nfsd_cache_bucket_find(rqstp->rq_xid, nn);
 	spin_lock(&b->cache_lock);
 	found = nfsd_cache_insert(b, rp, nn);
 	if (found != rp) {