netfilter: nf_tables: fix double-free on set expression from the error path

After copying the expression to the set element extension, release the
expression and reset the pointer to avoid a double-free from the error
path.

Fixes: 4094445229 ("netfilter: nf_tables: add elements with stateful expressions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -5133,6 +5133,7 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
 	if (expr) {
 		memcpy(nft_set_ext_expr(ext), expr, expr->ops->size);
 		kfree(expr);
+		expr = NULL;
 	}
 
 	trans = nft_trans_elem_alloc(ctx, NFT_MSG_NEWSETELEM, set);