apparmor: Use pointer to struct aa_label for lbs_cred

According to the implementations of cred_label() and set_cred_label(),
we should use pointer to struct aa_label for lbs_cred instead of struct
aa_task_ctx, this patch fixes it.

Fixes: bbd3662a ("Infrastructure management of the cred security blob")
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>