netfilter: nf_tables: drop unused 3rd argument from validate callback ops

Since commit a654de8f ("netfilter: nf_tables: fix chain dependency validation")
the validate() callback no longer needs the return pointer argument.

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