cpu/hotplug: Fix unused function warning

The cpuhp_is_ap_state() function is no longer called outside of the
CONFIG_SMP #ifdef section, causing a harmless warning:

kernel/cpu.c:129:13: error: 'cpuhp_is_ap_state' defined but not used [-Werror=unused-function]

This moves the function into the #ifdef to get a clean build again.

Fixes: 17a2f1ced028 ("cpu/hotplug: Merge cpuhp_bp_states and cpuhp_ap_states")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: https://lkml.kernel.org/r/20180315153829.3819606-1-arnd@arndb.de

kernel/cpu.c

@@ -126,15 +126,6 @@ struct cpuhp_step {
 static DEFINE_MUTEX(cpuhp_state_mutex);
 static struct cpuhp_step cpuhp_hp_states[];
 
-static bool cpuhp_is_ap_state(enum cpuhp_state state)
-{
-	/*
-	 * The extra check for CPUHP_TEARDOWN_CPU is only for documentation
-	 * purposes as that state is handled explicitly in cpu_down.
-	 */
-	return state > CPUHP_BRINGUP_CPU && state != CPUHP_TEARDOWN_CPU;
-}
-
 static struct cpuhp_step *cpuhp_get_step(enum cpuhp_state state)
 {
 	return cpuhp_hp_states + state;
@@ -235,6 +226,15 @@ err:
 }
 
 #ifdef CONFIG_SMP
+static bool cpuhp_is_ap_state(enum cpuhp_state state)
+{
+	/*
+	 * The extra check for CPUHP_TEARDOWN_CPU is only for documentation
+	 * purposes as that state is handled explicitly in cpu_down.
+	 */
+	return state > CPUHP_BRINGUP_CPU && state != CPUHP_TEARDOWN_CPU;
+}
+
 static inline void wait_for_ap_thread(struct cpuhp_cpu_state *st, bool bringup)
 {
 	struct completion *done = bringup ? &st->done_up : &st->done_down;