[INET_DIAG]: Introduce sk_diag_fill

To be called from inet_diag_get_exact, also rename inet_diag_fill to
inet_csk_diag_fill, for consistency with inet_twsk_diag_fill.

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