[SK_BUFF]: Introduce udp_hdr(), remove skb->h.uh

drivers/net/gianfar.c

drivers/net/ioc3-eth.c

drivers/net/mv643xx_eth.c

include/linux/skbuff.h

include/linux/udp.h

include/net/udplite.h

net/core/netpoll.c

net/core/pktgen.c

net/ipv4/udp.c

net/ipv6/udp.c

net/rxrpc/connection.c

net/rxrpc/transport.c