AppArmor: Add selfattr hooks

Add hooks for setselfattr and getselfattr. These hooks are not very
different from their setprocattr and getprocattr equivalents, and
much of the code is shared.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: John Johansen <john.johansen@canonical.com>
[PM: forward ported beyond v6.6 due merge window changes]
Signed-off-by: Paul Moore <paul@paul-moore.com>