netfilter: nf_tables: disable expression reduction infra

Either userspace or kernelspace need to pre-fetch keys inconditionally
before comparisons for this to work. Otherwise, register tracking data
is misleading and it might result in reducing expressions which are not
yet registers.

First expression is also guaranteed to be evaluated always, however,
certain expressions break before writing data to registers, before
comparing the data, leaving the register in undetermined state.

This patch disables this infrastructure by now.

Fixes: b2d30654 ("netfilter: nf_tables: do not reduce read-only expressions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>