userns: Teach inet_diag to work with user namespaces

Compute the user namespace of the socket that we are replying to
and translate the kuids of reported sockets into that user namespace.

Cc: Andrew Vagin <avagin@openvz.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>