[NETFILTER]: annotate rest of nf_nat_* with const

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>

diff --cc net/ipv4/netfilter/nf_nat_core.c
Simple merge

diff --cc net/ipv4/netfilter/nf_nat_snmp_basic.c
Simple merge

diff --cc net/ipv4/netfilter/nf_nat_standalone.c
Simple merge