netfilter.mk: add version conditional for nf_nat_ipv4,6

The upstream linux commit is 3bf195ae ("netfilter: nat: merge
nf_nat_ipv4,6 into nat core").  It was included since linux 5.1

Signed-off-by: Yousong Zhou <[email protected]>

include/netfilter.mk

@@ -185,8 +185,8 @@ $(eval $(call nf_add,IPT_IPV6_EXTRA,CONFIG_IP6_NF_MATCH_RT, $(P_V6)ip6t_rt))
 # kernel only
 $(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT, $(P_XT)nf_nat),))
 $(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT_REDIRECT, $(P_XT)nf_nat_redirect),))
-$(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT_IPV4, $(P_V4)nf_nat_ipv4),))
-$(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT6,CONFIG_NF_NAT_IPV6, $(P_V6)nf_nat_ipv6),))
+$(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT_IPV4, $(P_V4)nf_nat_ipv4, lt 5.1)))
+$(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT6,CONFIG_NF_NAT_IPV6, $(P_V6)nf_nat_ipv6, lt 5.1)))
 
 $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT,CONFIG_NETFILTER_XT_NAT, $(P_XT)xt_nat),))
 $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT,CONFIG_IP_NF_NAT, $(P_V4)iptable_nat),))