netfilter: nft_lookup: only cancel tracking for clobbered dregs

In most cases, nft_lookup will be read-only, i.e. won't clobber
registers.  In case of map, we need to cancel the registers that will
see stores.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>