netfilter: nf_tables: drop map element references from preparation phase

[ Upstream commit 628bd3e49cba1c066228e23d71a852c23e26da73 ]

set .destroy callback releases the references to other objects in maps.
This is very late and it results in spurious EBUSY errors. Drop refcount
from the preparation phase instead, update set backend not to drop
reference counter from set .destroy path.

Exceptions: NFT_TRANS_PREPARE_ERROR does not require to drop the
reference counter because the transaction abort path releases the map
references for each element since the set is unbound. The abort path
also deals with releasing reference counter for new elements added to
unbound sets.

Fixes: 591054469b ("netfilter: nf_tables: revisit chain/object refcounting from elements")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/net/netfilter/nf_tables.h

@@ -382,7 +382,8 @@ struct nft_set_ops {
 	int				(*init)(const struct nft_set *set,
 						const struct nft_set_desc *desc,
 						const struct nlattr * const nla[]);
-	void				(*destroy)(const struct nft_set *set);
+	void				(*destroy)(const struct nft_ctx *ctx,
+						   const struct nft_set *set);
 	void				(*gc_init)(const struct nft_set *set);
 
 	unsigned int			elemsize;
@@ -686,6 +687,8 @@ void *nft_set_elem_init(const struct nft_set *set,
 			u64 timeout, u64 expiration, gfp_t gfp);
 void nft_set_elem_destroy(const struct nft_set *set, void *elem,
 			  bool destroy_expr);
+void nf_tables_set_elem_destroy(const struct nft_ctx *ctx,
+				const struct nft_set *set, void *elem);
 
 /**
  *	struct nft_set_gc_batch_head - nf_tables set garbage collection batch