contrib/check-config.sh: fix NF_NAT_IPV4

CONFIG_NF_NAT_IPV4 was removed in kernel commit 3bf195ae6037e310,
which made its way into v5.1-rc1. The functionality is now under
NF_NAT which we already check for.

Make the check for NF_NAT_IPV4 conditional.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

contrib/check-config.sh

@@ -196,7 +196,7 @@ flags=(
 	CGROUPS CGROUP_CPUACCT CGROUP_DEVICE CGROUP_FREEZER CGROUP_SCHED CPUSETS MEMCG
 	KEYS
 	VETH BRIDGE BRIDGE_NETFILTER
-	NF_NAT_IPV4 IP_NF_FILTER IP_NF_TARGET_MASQUERADE
+	IP_NF_FILTER IP_NF_TARGET_MASQUERADE
 	NETFILTER_XT_MATCH_{ADDRTYPE,CONNTRACK,IPVS}
 	IP_NF_NAT NF_NAT NF_NAT_NEEDED
 
@@ -208,6 +208,10 @@ if [ "$kernelMajor" -lt 4 ] || ([ "$kernelMajor" -eq 4 ] && [ "$kernelMinor" -lt
 	check_flags DEVPTS_MULTIPLE_INSTANCES
 fi
 
+if [ "$kernelMajor" -lt 5 ] || [ "$kernelMajor" -eq 5 -a "$kernelMinor" -le 1 ]; then
+	check_flags NF_NAT_IPV4
+fi
+
 echo
 
 echo 'Optional Features:'