SUNRPC: Convert svc_xprt_release() to the release_pages() API

Instead of invoking put_page() one-at-a-time, pass the "response"
portion of rq_pages directly to release_pages() to reduce the number
of times each nfsd thread invokes a page allocator API.

Since svc_xprt_release() is not invoked while a client is waiting
for an RPC Reply, this is not expected to directly impact mean
request latencies on a lightly or moderately loaded server. However
as workload intensity increases, I expect somewhat better
scalability: the same number of server threads should be able to
handle more work.
Reviewed-by: Calum Mackay <calum.mackay@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc.c

@@ -878,13 +878,12 @@ EXPORT_SYMBOL_GPL(svc_rqst_replace_page);
  */
 void svc_rqst_release_pages(struct svc_rqst *rqstp)
 {
-	while (rqstp->rq_next_page != rqstp->rq_respages) {
-		struct page **pp = --rqstp->rq_next_page;
+	int i, count = rqstp->rq_next_page - rqstp->rq_respages;
 
-		if (*pp) {
-			put_page(*pp);
-			*pp = NULL;
-		}
+	if (count) {
+		release_pages(rqstp->rq_respages, count);
+		for (i = 0; i < count; i++)
+			rqstp->rq_respages[i] = NULL;
 	}
 }