Rename class_destroy to avoid namespace conflicts.

We're seeing increasing namespace conflicts between the global
class_destroy() function declared in linux/device.h, and the private
function in the SELinux core code.  This patch renames the SELinux
function to cls_destroy() to avoid this conflict.

Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>