netfilter: nf_tables: set last expression in register tracking area

nft_rule_for_each_expr() sets on last to nft_rule_last(), however, this
is coming after track.last field is set on.

Use nft_expr_last() to set track.last accordingly.

Fixes: 12e4ecfa ("netfilter: nf_tables: add register tracking infrastructure")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -8299,7 +8299,7 @@ static int nf_tables_commit_chain_prepare(struct net *net, struct nft_chain *cha
 			return -ENOMEM;
 
 		size = 0;
-		track.last = last;
+		track.last = nft_expr_last(rule);
 		nft_rule_for_each_expr(expr, last, rule) {
 			track.cur = expr;