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 <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

include/linux/percpu_counter.h

···
 	return (fbc->counters != NULL);
 }
 
-#else
+#else /* !CONFIG_SMP */
 
 struct percpu_counter {
 	s64 count;