netfilter: nf_tables: simplify NLM_F_CREATE handling

* From nf_tables_newchain(), codepath provides context that allows us to
  infer if we are updating a chain (in that case, no module autoload is
  required) or adding a new one (then, module autoload is indeed
  needed).
* We only need it in one single spot in nf_tables_newrule().
* Not needed for nf_tables_newset() at all.

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