netfilter: nf_tables: add helpers to schedule objects deletion

This patch refactor the code to schedule objects deletion.
They are useful in follow-up patches.

In order to be able to use these new helper functions in all the code,
they are placed in the top of the file, with all the dependant functions
and symbols.

nft_rule_disactivate_next has been renamed to nft_rule_deactivate.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>