NFSv4.2: Fix warning "variable ‘stateids’ set but not used"

Replace it with a test for whether or not the sent a stateid in violation
of what we asked for.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs42xdr.c

@@ -330,13 +330,21 @@ static int decode_write_response(struct xdr_stream *xdr,
 				 struct nfs42_write_res *res)
 {
 	__be32 *p;
-	int stateids;
 
 	p = xdr_inline_decode(xdr, 4 + 8 + 4);
 	if (unlikely(!p))
 		goto out_overflow;
 
-	stateids = be32_to_cpup(p++);
+	/*
+	 * We never use asynchronous mode, so warn if a server returns
+	 * a stateid.
+	 */
+	if (unlikely(*p != 0)) {
+		pr_err_once("%s: server has set unrequested "
+				"asynchronous mode\n", __func__);
+		return -EREMOTEIO;
+	}
+	p++;
 	p = xdr_decode_hyper(p, &res->count);
 	res->verifier.committed = be32_to_cpup(p);
 	return decode_verifier(xdr, &res->verifier.verifier);