net: Check the correct namespace when spoofing pid over SCM_RIGHTS

commit d661684cf6820331feae71146c35da83d794467e upstream.

This is a security bug.

The follow-up will fix nsproxy to discourage this type of issue from
happening again.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>