netfilter: nf_tables: refactor rule deletion helper

This helper function always schedule the rule to be removed in the following
transaction.
In follow-up patches, it is interesting to handle separately the logic of rule
activation/disactivation from the transaction mechanism.

So, this patch simply splits the original nf_tables_delrule_one() in two
functions, allowing further control.

While at it, for the sake of homigeneize the function naming scheme, let's
rename nf_tables_delrule_one() to nft_delrule().

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