netfilter: nft_compat: don't use refcount_inc on newly allocated entry

When I moved the refcount to refcount_t type I missed the fact that
refcount_inc() will result in use-after-free warning with
CONFIG_REFCOUNT_FULL=y builds.

The correct fix would be to init the reference count to 1 at allocation
time, but, unfortunately we cannot do this, as we can't undo that
in case something else fails later in the batch.

So only solution I see is to special-case the 'new entry' condition
and replace refcount_inc() with a "delayed" refcount_set(1) in this case,
as done here.

The .activate callback can be removed to simplify things, we only
need to make sure that deactivate() decrements/unlinks the entry
from the list at end of transaction phase (commit or abort).

Fixes: 12c44aba ("netfilter: nft_compat: use refcnt_t type for nft_xt reference count")
Reported-by: Jordan Glover <Golden_Miller83@protonmail.ch>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>