netfilter: nf_tables: move base hook annotation to init helper

coverity scanner says:
2187  if (nft_is_base_chain(chain)) {
vvv   CID 1505166:  Memory - corruptions  (UNINIT)
vvv   Using uninitialized value "basechain".
2188  basechain->ops.hook_ops_type = NF_HOOK_OP_NF_TABLES;

... I don't see how nft_is_base_chain() can evaluate to true
while basechain pointer is garbage.

However, it seems better to place the NF_HOOK_OP_NF_TABLES annotation
in nft_basechain_hook_init() instead.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1505166 ("Memory - corruptions")
Fixes: 65b8b7bfc5284f ("netfilter: annotate nf_tables base hook ops")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>