netfilter: nft_dynset: support for element deletion

This patch implements the delete operation from the ruleset.

It implements a new delete() function in nft_set_rhash. It is simpler
to use than the already existing remove(), because it only takes the set
and the key as arguments, whereas remove() expects a full
nft_set_elem structure.

Signed-off-by: Ander Juaristi <a@juaristi.eus>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -636,6 +636,7 @@ enum nft_lookup_attributes {
 enum nft_dynset_ops {
 	NFT_DYNSET_OP_ADD,
 	NFT_DYNSET_OP_UPDATE,
+	NFT_DYNSET_OP_DELETE,
 };
 
 enum nft_dynset_flags {