libmnl: fix valgrind warnings about padding

[Stephen Hemminger <shemminger@vyatta.com>]

When using  mnl_nlmsg_put_extra_header() it pads out the addtional
header but only zeros the original size not the padded value. Which
cause valgrind to complain about sendto() with unitialized byte.

--- a/src/nlmsg.c
+++ b/src/nlmsg.c
@@ -105,8 +105,9 @@ void *
 mnl_nlmsg_put_extra_header(struct nlmsghdr *nlh, size_t size)
 {
        char *ptr = (char *)nlh + nlh->nlmsg_len;
-       nlh->nlmsg_len += MNL_ALIGN(size);
-       memset(ptr, 0, size);
+       size_t len = MNL_ALIGN(size);
+       nlh->nlmsg_len += len;
+       memset(ptr, 0, len);
        return ptr;
 }
 EXPORT_SYMBOL(mnl_nlmsg_put_extra_header);