selinux: fix a type cast problem in cred_init_security()

In the process of removing an explicit type cast to preserve a cred
const qualifier in cred_init_security() we ran into a problem where
the task_struct::real_cred field is defined with the "__rcu"
attribute but the selinux_cred() function parameter is not, leading
to a sparse warning:

  security/selinux/hooks.c:216:36: sparse: sparse:
    incorrect type in argument 1 (different address spaces)
    @@     expected struct cred const *cred
    @@     got struct cred const [noderef] __rcu *real_cred

As we don't want to add the "__rcu" attribute to the selinux_cred()
parameter, we're going to add an explicit cast back to
cred_init_security().

Fixes: b084e189

 ("selinux: simplify cred_init_security")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>