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>