netfilter: nft_bitwise: fix register tracking

At the end of `nft_bitwise_reduce`, there is a loop which is intended to
update the bitwise expression associated with each tracked destination
register.  However, currently, it just updates the first register
repeatedly.  Fix it.

Fixes: 34cc9e52

 ("netfilter: nf_tables: cancel tracking for clobbered destination registers")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>