rcupdate: fix comment

This comment caused some consternation during fastcall removal.  Make it
truthful.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/rcupdate.c

@@ -56,7 +56,10 @@ static atomic_t rcu_barrier_cpu_count;
 static DEFINE_MUTEX(rcu_barrier_mutex);
 static struct completion rcu_barrier_completion;
 
-/* Because of FASTCALL declaration of complete, we use this wrapper */
+/*
+ * Awaken the corresponding synchronize_rcu() instance now that a
+ * grace period has elapsed.
+ */
 static void wakeme_after_rcu(struct rcu_head  *head)
 {
 	struct rcu_synchronize *rcu;