netfilter: nft_fib: add reduce support

The fib expression stores to a register, so we can't add empty stub.
Check that the register that is being written is in fact redundant.

In most cases, this is expected to cancel tracking as re-use is
unlikely.

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