hrtimer: Add a missing bracket and hide `migration_base' on !SMP

The recent change to avoid taking the expiry lock when a timer is currently
migrated missed to add a bracket at the end of the if statement leading to
compile errors.  Since that commit the variable `migration_base' is always
used but it is only available on SMP configuration thus leading to another
compile error.  The changelog says "The timer base and base->cpu_base
cannot be NULL in the code path", so it is safe to limit this check to SMP
configurations only.

Add the missing bracket to the if statement and hide `migration_base'
behind CONFIG_SMP bars.

[ tglx: Mark the functions inline ... ]

Fixes: 68b2c8c ("hrtimer: Don't take expiry_lock when timer is currently migrated")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190904145527.eah7z56ntwobqm6j@linutronix.de

kernel/time/hrtimer.c

@@ -140,6 +140,11 @@ static struct hrtimer_cpu_base migration_cpu_base = {
 
 #define migration_base	migration_cpu_base.clock_base[0]
 
+static inline bool is_migration_base(struct hrtimer_clock_base *base)
+{
+	return base == &migration_base;
+}
+
 /*
  * We are using hashed locking: holding per_cpu(hrtimer_bases)[n].lock
  * means that all timers which are tied to this base via timer->base are
@@ -264,6 +269,11 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base,
 
 #else /* CONFIG_SMP */
 
+static inline bool is_migration_base(struct hrtimer_clock_base *base)
+{
+	return false;
+}
+
 static inline struct hrtimer_clock_base *
 lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
 {
@@ -1221,7 +1231,7 @@ void hrtimer_cancel_wait_running(const struct hrtimer *timer)
 	 * Just relax if the timer expires in hard interrupt context or if
 	 * it is currently on the migration base.
 	 */
-	if (!timer->is_soft || base == &migration_base)
+	if (!timer->is_soft || is_migration_base(base)) {
 		cpu_relax();
 		return;
 	}