SUNRPC: Fix up typo in xdr_init_decode()

We already know that the head buffer and page are empty, so if there is
any data, it is in the tail.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xdr.c

@@ -1060,7 +1060,7 @@ void xdr_init_decode(struct xdr_stream *xdr, struct xdr_buf *buf, __be32 *p,
 	else if (buf->page_len != 0)
 		xdr_set_page_base(xdr, 0, buf->len);
 	else
-		xdr_set_iov(xdr, buf->head, buf->len);
+		xdr_set_iov(xdr, buf->tail, buf->len);
 	if (p != NULL && p > xdr->p && xdr->end >= p) {
 		xdr->nwords -= p - xdr->p;
 		xdr->p = p;