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 fc439d9489479411fbf9bbbec2c768df89e85503]

Change-Id: I13515e83fb76d4729f00047f9eb142c929390fb2
Reported-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>