netfilter: nf_tables: Simplify chain netdev notifier

With conditional chain deletion gone, callback code simplifies: Instead
of filling an nft_ctx object, just pass basechain to the per-chain
function. Also plain list_for_each_entry() is safe now.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>