Use inline function instead of macro.

This means we won't get any warnings about unused
variables - they _are_ used in some configurations.

include/linux/cpu.h

@@ -74,7 +74,7 @@ int cpu_down(unsigned int cpu);
 #define hotcpu_notifier(fn, pri)
 
 /* CPUs don't go offline once they're online w/o CONFIG_HOTPLUG_CPU */
-#define cpu_is_offline(cpu) 0
+static inline int cpu_is_offline(int cpu) { return 0; }
 #endif
 
 #endif /* _LINUX_CPU_H_ */