netfilter: nft_meta: use raw_smp_processor_id()

Using smp_processor_id() triggers warnings with PREEMPT_RCU. There is no
point in disabling preemption since we only collect the numeric value,
so use raw_smp_processor_id() instead.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>