netfilter: nf_tables: warn when expr implements only one of activate/deactivate

->destroy is only allowed to free data, or do other cleanups that do not
have side effects on other state, such as visibility to other netlink
requests.

Such things need to be done in ->deactivate.
As a transaction can fail, we need to make sure we can undo such
operations, therefore ->activate() has to be provided too.

So print a warning and refuse registration if expr->ops provides
only one of the two operations.

v2: fix nft_expr_check_ops to not repeat same check twice (Jones Desougi)

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