net: Put fl4_* macros to struct flowi4 and use them again.

include/net/flow.h

include/net/route.h

net/dccp/ipv4.c

net/ipv4/icmp.c

net/ipv4/inet_connection_sock.c

net/ipv4/ip_output.c

net/ipv4/netfilter/nf_nat_standalone.c

net/ipv4/raw.c

net/ipv4/syncookies.c

net/ipv4/udp.c

net/ipv4/xfrm4_policy.c

net/sctp/protocol.c