[PATCH] kNFSd: Fix bug in svc_pushback_unused_pages that occurs on zero byte NFS read

svc_pushback_unused_pages must be ready of the possibility that
no pages were allocated or will need to be pushed back.

include/linux/sunrpc/svc.h

@@ -196,14 +196,14 @@ static inline void svc_pushback_allpages(struct svc_rqst *rqstp)
 
 static inline void svc_pushback_unused_pages(struct svc_rqst *rqstp)
 {
-        while (rqstp->rq_resused) {
+	while (rqstp->rq_resused &&
+	       rqstp->rq_res.pages != &rqstp->rq_respages[rqstp->rq_resused]) {
+
 		if (rqstp->rq_respages[--rqstp->rq_resused] != NULL) {
 			rqstp->rq_argpages[rqstp->rq_arghi++] =
 				rqstp->rq_respages[rqstp->rq_resused];
 			rqstp->rq_respages[rqstp->rq_resused] = NULL;
 		}
-		if (rqstp->rq_res.pages == &rqstp->rq_respages[rqstp->rq_resused])
-			break;
 	}
 }