netfilter: nft_inner: Use nested-BH locking for nft_pcpu_tun_ctx

nft_pcpu_tun_ctx 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.

Make a struct with a nft_inner_tun_ctx member (original
nft_pcpu_tun_ctx) and a local_lock_t 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: Pablo Neira Ayuso <pablo@netfilter.org>