netfilter: nft_set: remove indirection from update API call

This stems from a time when sets and nft_dynset resided in different kernel
modules.  We can replace this with a direct call.

We could even remove both ->update and ->delete, given its only
supported by rhashtable, but on the off-chance we'll see runtime
add/delete for other types or a new set type keep that as-is for now.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>