lsm: use unrcu_pointer() for current->cred in security_init()

lsm: use unrcu_pointer() for current->cred in security_init()

We need to directly allocate the cred's LSM state for the initial task
when we initialize the LSM framework.  Unfortunately, this results in a
RCU related type mismatch, use the unrcu_pointer() macro to handle this
a bit more elegantly.

The explicit type casting still remains as we need to work around the
constification of current->cred in this particular case.

Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>