rcu: Identify dyntick-idle CPUs on first force_quiescent_state() pass

Fixes and workarounds for a number of issues (for example, that in
df4012edc) make it safe to once again detect dyntick-idle CPUs on the
first pass of force_quiescent_state(), so this commit makes that change.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index aab9ed5..69bb372 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -607,7 +607,7 @@
 static int dyntick_save_progress_counter(struct rcu_data *rdp)
 {
 	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
-	return 0;
+	return (rdp->dynticks_snap & 0x1) == 0;
 }
 
 /*