rcu/nocb: Allow de-offloading rdp leader

The only thing that prevented an rdp leader from being de-offloaded was
the nocb_bypass_timer that used to lock the nocb_lock of the rdp leader.

If an rdp gets de-offloaded, it will subtlely ignore rcu_nocb_lock()
calls and do its job in the timer unsafely.  Worse yet:  If it gets
re-offloaded in the middle of the timer, rcu_nocb_unlock() would try to
unlock, leaving it imbalanced.

Now that the nocb_bypass_timer doesn't use the nocb_lock anymore,
de-offloading the rdp leader is now safe.  This commit therefore allows
the rdp leader to be de-offloaded.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -2489,10 +2489,6 @@ int rcu_nocb_cpu_deoffload(int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	int ret = 0;
 
-	if (rdp == rdp->nocb_gp_rdp) {
-		pr_info("Can't deoffload an rdp GP leader (yet)\n");
-		return -EINVAL;
-	}
 	mutex_lock(&rcu_state.barrier_mutex);
 	cpus_read_lock();
 	if (rcu_rdp_is_offloaded(rdp)) {