netfilter: nf_tables: use kzalloc for hook allocation

KMSAN reports unitialized variable when registering the hook,
   reg->hook_ops_type == NF_HOOK_OP_BPF)
        ~~~~~~~~~~~ undefined

This is a small structure, just use kzalloc to make sure this
won't happen again when new fields get added to nf_hook_ops.

Fixes: 7b4b2fa3 ("netfilter: annotate nf_tables base hook ops")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>