netfilter: nf_tables: honor NLM_F_EXCL flag in set element insertion

If the NLM_F_EXCL flag is set, then new elements that clash with an
existing one return EEXIST. In case you try to add an element whose
data area differs from what we have, then this returns EBUSY. If no
flag is specified at all, then this returns success to userspace.

This patch also update the set insert operation so we can fetch the
existing element that clashes with the one you want to add, we need
this to make sure the element data doesn't differ.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -251,7 +251,8 @@ struct nft_set_ops {
 
 	int				(*insert)(const struct net *net,
 						  const struct nft_set *set,
-						  const struct nft_set_elem *elem);
+						  const struct nft_set_elem *elem,
+						  struct nft_set_ext **ext);
 	void				(*activate)(const struct net *net,
 						    const struct nft_set *set,
 						    const struct nft_set_elem *elem);