net: diag: support v4mapped sockets in inet_diag_find_one_icsk()

Lorenzo reported that we could not properly find v4mapped sockets
in inet_diag_find_one_icsk(). This patch fixes the issue.

[Cherry-pick of net 7c1306723ee916ea9f1fa7d9e4c7a6d029ca7aaf]

Change-Id: If71ddbc2f082e708e5fa9d60f5c08702a09e2884
Reported-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>