selinux: Use current_security() when possible

There were a number of places using the following code pattern:

  struct cred *cred = current_cred();
  struct task_security_struct *tsec = cred->security;

... which were simplified to the following:

  struct task_security_struct *tsec = current_security();

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>