netfilter: nf_tables: fix null deref due to zeroed list head

In nf_tables_updtable, if nf_tables_table_enable returns an error,
nft_trans_destroy is called to free the transaction object.

nft_trans_destroy() calls list_del(), but the transaction was never
placed on a list -- the list head is all zeroes, this results in
a null dereference:

BUG: KASAN: null-ptr-deref in nft_trans_destroy+0x26/0x59
Call Trace:
 nft_trans_destroy+0x26/0x59
 nf_tables_newtable+0x4bc/0x9bc
 [..]

Its sane to assume that nft_trans_destroy() can be called
on the transaction object returned by nft_trans_alloc(), so
make sure the list head is initialised.

Fixes: 55dd6f93 ("netfilter: nf_tables: use new transaction infrastructure to handle table")
Reported-by: mingi cho <mgcho.minic@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -153,6 +153,7 @@ static struct nft_trans *nft_trans_alloc_gfp(const struct nft_ctx *ctx,
 	if (trans == NULL)
 		return NULL;
 
+	INIT_LIST_HEAD(&trans->list);
 	trans->msg_type = msg_type;
 	trans->ctx	= *ctx;