inet_diag: Introduce the inet socket dumping routine

The existing inet_csk_diag_fill dumps the inet connection sock info
into the netlink inet_diag_message. Prepare this routine to be able
to dump only the inet_sock part of a socket if the icsk part is missing.

This will be used by UDP diag module when dumping UDP sockets.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>