Shut up per_cpu_ptr() on UP

Currently per_cpu_ptr() doesn't really do anything with 'cpu' in the UP case. This is problematic in the cases where this is the only place the variable is referenced:

CC kernel/workqueue.o
kernel/workqueue.c: In function `current_is_keventd':
kernel/workqueue.c:460: warning: unused variable `cpu'

How about something like this?

index 5451eb1..fb8d2d2 100644
@@ -38,7 +38,7 @@ extern void free_percpu(const void *);
 
 #else /* CONFIG_SMP */
 
-#define per_cpu_ptr(ptr, cpu) (ptr)
+#define per_cpu_ptr(ptr, cpu) ({ (void)(cpu); (ptr); })
 
 static inline void *__alloc_percpu(size_t size, size_t align)
 {