nfsd4: fix xdr decoding of large non-write compounds

This fixes a regression from 24750082
"nfsd4: fix decoding of compounds across page boundaries".  The previous
code was correct: argp->pagelist is initialized in
nfs4svc_deocde_compoundargs to rqstp->rq_arg.pages, and is therefore a
pointer to the page *after* the page we are currently decoding.

The reason that patch nevertheless fixed a problem with decoding
compounds containing write was a bug in the write decoding introduced by
5a80a54d "nfsd4: reorganize write
decoding", after which write decoding no longer adhered to the rule that
argp->pagelist point to the next page.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -141,8 +141,8 @@ xdr_error:					\
 
 static void next_decode_page(struct nfsd4_compoundargs *argp)
 {
-	argp->pagelist++;
 	argp->p = page_address(argp->pagelist[0]);
+	argp->pagelist++;
 	if (argp->pagelen < PAGE_SIZE) {
 		argp->end = argp->p + (argp->pagelen>>2);
 		argp->pagelen = 0;
@@ -1229,6 +1229,7 @@ nfsd4_decode_write(struct nfsd4_compoundargs *argp, struct nfsd4_write *write)
 		len -= pages * PAGE_SIZE;
 
 		argp->p = (__be32 *)page_address(argp->pagelist[0]);
+		argp->pagelist++;
 		argp->end = argp->p + XDR_QUADLEN(PAGE_SIZE);
 	}
 	argp->p += XDR_QUADLEN(len);