No need for two structs, follow the new inet_sock layout style.

Also introduce inet_sk_copy_descendant, to copy just the inet_sock
descendant specific area from one sock to another.
Signed-off-by: Arnaldo Carvalho de Melo <acme@conectiva.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>