[NETFILTER]: nf_nat: properly use RCU API for nf_nat_protos array

Replace preempt_{enable,disable} based RCU by proper use of the
RCU API and add missing rcu_read_lock/rcu_read_unlock calls in
paths used outside of packet processing context (nfnetlink_conntrack).

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>