netfilter: nf_tables: pass ctx to nf_tables_expr_destroy()

nft_set_elem_destroy() can be called from call_rcu context. Annotate
netns and table in set object so we can populate the context object.
Moreover, pass context object to nf_tables_set_elem_destroy() from the
commit phase, since it is already available from there.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>