inet_diag: factorize code in new inet_diag_msg_common_fill() helper

Now the three type of sockets share a common base, we can factorize
code in inet_diag_msg_common_fill().

inet_diag_entry no longer requires saddr_storage & daddr_storage
and the extra copies.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>