netfilter: nf_tables: Reintroduce shortened deletion notifications

Restore commit 28339b21 ("netfilter: nf_tables: do not send complete
notification of deletions") and fix it:

- Avoid upfront modification of 'event' variable so the conditionals
  become effective.
- Always include NFTA_OBJ_TYPE attribute in object notifications, user
  space requires it for proper deserialisation.
- Catch DESTROY events, too.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>