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

[ Upstream commit 0ef235c71755c5f36c50282fcf2d7d08709be344 ]

->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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>