sunrpc: do array overrun check in svc_recv before allocating pages

There's little point in waiting until after we allocate all of the pages
to see if we're going to overrun the array. In the event that this
calculation is really off we could end up scribbling over a bunch of
memory and make it tougher to debug.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -601,6 +601,7 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
 
 	/* now allocate needed pages.  If we get a failure, sleep briefly */
 	pages = (serv->sv_max_mesg + PAGE_SIZE) / PAGE_SIZE;
+	BUG_ON(pages >= RPCSVC_MAXPAGES);
 	for (i = 0; i < pages ; i++)
 		while (rqstp->rq_pages[i] == NULL) {
 			struct page *p = alloc_page(GFP_KERNEL);
@@ -615,7 +616,6 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
 			rqstp->rq_pages[i] = p;
 		}
 	rqstp->rq_pages[i++] = NULL; /* this might be seen in nfs_read_actor */
-	BUG_ON(pages >= RPCSVC_MAXPAGES);
 
 	/* Make arg->head point to first page and arg->pages point to rest */
 	arg = &rqstp->rq_arg;