[INET_DIAG]: Introduce inet_twsk_diag_dump & inet_twsk_diag_fill

To properly dump TIME_WAIT sockets and to reduce complexity a bit by
having per socket class accessor routines.

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>