nfqueue: enable to set skb->priority

This is a follow up of the previous patch that enables to get
skb->priority. It's now posssible to set it also.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Florian Westphal <fw@strlen.de>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>