netfilter: nf_tables: restore context for expression destructors

In order to fix set destruction notifications and get rid of unnecessary
members in private data structures, pass the context to expressions'
destructor functions again.

In order to do so, replace various members in the nft_rule_trans structure
by the full context.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>