percpu_counter.h: comment code for better readability

Help people reading the percpu_counter code, to notice the ifdef
else statement that seperates CONFIG_SMP.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/linux/percpu_counter.h

@@ -83,7 +83,7 @@ static inline int percpu_counter_initialized(struct percpu_counter *fbc)
 	return (fbc->counters != NULL);
 }
 
-#else
+#else /* !CONFIG_SMP */
 
 struct percpu_counter {
 	s64 count;