[NET]: netfilter checksum annotations

include/linux/netfilter.h

include/linux/netfilter_ipv4/ip_conntrack_proto_gre.h

net/ipv4/netfilter/ip_nat_core.c

net/ipv4/netfilter/ip_nat_helper.c

net/ipv4/netfilter/ip_nat_proto_gre.c

net/ipv4/netfilter/ip_nat_proto_icmp.c

net/ipv4/netfilter/ip_nat_proto_tcp.c

net/ipv4/netfilter/ip_nat_proto_udp.c

net/ipv4/netfilter/ipt_ECN.c

net/ipv4/netfilter/ipt_TCPMSS.c

net/ipv4/netfilter/ipt_TOS.c

net/ipv4/netfilter/ipt_TTL.c

net/netfilter/core.c