apparmor: drop cred_ctx and reference the label directly

apparmor: drop cred_ctx and reference the label directly

With the task domain change information now stored in the task->security
context, the cred->security context only stores the label. We can get
rid of the cred_ctx and directly reference the label, removing a layer
of indirection, and unneeded extra allocations.

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