netfilter: nft_set_pipapo: Use nested-BH locking for nft_pipapo_scratch

nft_pipapo_scratch is a per-CPU variable and relies on disabled BH for
its locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
this data structure requires explicit locking.

Add a local_lock_t to the data structure and use local_lock_nested_bh() for
locking. This change adds only lockdep coverage and does not alter the
functional behaviour for !PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Florian Westphal <fw@strlen.de>