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.

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>