apparmor: Ensure apparmor does not mediate kernel based sockets

Currently apparmor makes the assumption that kernel sockets are unmediated
because mediation is only done against tasks that have a profile attached.
Ensure we never get in a situation where a kernel socket is being mediated
by tagging the sk_security field for kernel sockets.

Signed-off-by: John Johansen <john.johansen@canonical.com>