netlink: Fix nla_parse_nested_compat() to call nla_parse() directly

The purpose of nla_parse_nested_compat() is to parse attributes which
contain a struct followed by a stream of nested attributes.  So far,
it called nla_parse_nested() to parse the stream of nested attributes
which was wrong, as nla_parse_nested() expects a container attribute
as data which holds the attribute stream.  It needs to call
nla_parse() directly while pointing at the next possible alignment
point after the struct in the beginning of the attribute.

With this patch, I can no longer reproduce the reported leftover
warnings.
Signed-off-by: NThomas Graf <tgraf@suug.ch>
Acked-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -772,12 +772,13 @@ static inline int __nla_parse_nested_compat(struct nlattr *tb[], int maxtype,
 					    const struct nla_policy *policy,
 					    int len)
 {
-	if (nla_len(nla) < len)
+	int nested_len = nla_len(nla) - NLA_ALIGN(len);
+
+	if (nested_len < 0)
 		return -1;
-	if (nla_len(nla) >= NLA_ALIGN(len) + sizeof(struct nlattr))
-		return nla_parse_nested(tb, maxtype,
-					nla_data(nla) + NLA_ALIGN(len),
-					policy);
+	if (nested_len >= nla_attr_size(0))
+		return nla_parse(tb, maxtype, nla_data(nla) + NLA_ALIGN(len),
+				 nested_len, policy);
 	memset(tb, 0, sizeof(struct nlattr *) * (maxtype + 1));
 	return 0;
 }