netfilter: nf_tables: make nft_set_do_lookup available unconditionally

This function was added for retpoline mitigation and is replaced by a
static inline helper if mitigations are not enabled.

Enable this helper function unconditionally so next patch can add a lookup
restart mechanism to fix possible false negatives while transactions are
in progress.

Adding lookup restarts in nft_lookup_eval doesn't work as nft_objref would
then need the same copypaste loop.

This patch is separate to ease review of the actual bug fix.

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