SUNRPC: constify copied structure

The empty_iov structure is only copied into another structure,
so make it const.

The opportunity for this change was found using Coccinelle.
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xdr.c

@@ -1079,7 +1079,7 @@ void xdr_enter_page(struct xdr_stream *xdr, unsigned int len)
 }
 EXPORT_SYMBOL_GPL(xdr_enter_page);
 
-static struct kvec empty_iov = {.iov_base = NULL, .iov_len = 0};
+static const struct kvec empty_iov = {.iov_base = NULL, .iov_len = 0};
 
 void
 xdr_buf_from_iov(struct kvec *iov, struct xdr_buf *buf)