selinux: rename task_security_struct to cred_security_struct

selinux: rename task_security_struct to cred_security_struct

Before Linux had cred structures, the SELinux task_security_struct was
per-task and although the structure was switched to being per-cred
long ago, the name was never updated. This change renames it to
cred_security_struct to avoid confusion and pave the way for the
introduction of an actual per-task security structure for SELinux. No
functional change.

Cc: stable@vger.kernel.org
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>