CRED: Wrap access to SELinux's task SID

Wrap access to SELinux's task SID, using task_sid() and current_sid() as
appropriate.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NJames Morris <jmorris@namei.org>
Signed-off-by: NJames Morris <jmorris@namei.org>