[NET]: Annotate callers of csum_fold() in net/*

net/core/datagram.c

net/core/dev.c

net/core/netpoll.c

net/core/skbuff.c

net/ipv4/icmp.c

net/ipv4/igmp.c

net/ipv4/ip_gre.c

net/ipv4/ip_output.c

net/ipv4/ipmr.c

net/ipv4/ipvs/ip_vs_core.c

net/ipv4/netfilter.c

net/ipv4/tcp.c

net/sunrpc/socklib.c