NFS: Fix hdrlen calculation in NFSv4's decode_read()

When computing the length of the header, be sure to include the
four octets consumed by "count".
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4xdr.c

@@ -4475,7 +4475,7 @@ static int decode_read(struct xdr_stream *xdr, struct rpc_rqst *req, struct nfs_
 		goto out_overflow;
 	eof = be32_to_cpup(p++);
 	count = be32_to_cpup(p);
-	hdrlen = (u8 *) p - (u8 *) iov->iov_base;
+	hdrlen = (u8 *) xdr->p - (u8 *) iov->iov_base;
 	recvd = req->rq_rcv_buf.len - hdrlen;
 	if (count > recvd) {
 		dprintk("NFS: server cheating in read reply: "