audit: Properly set the origin port id of audit messages.

For user generated audit messages set the portid field in the netlink
header to the netlink port where the user generated audit message came
from.  Reporting the process id in a port id field was just nonsense.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>