For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and
CONFIG_RCU_LAZY=y, the following scenarios will trigger WARN_ON_ONCE()
in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended() functions:

        CPU2                                               CPU11
kthread
rcu_nocb_cb_kthread                                       ksys_write
rcu_do_batch                                              vfs_write
rcu_torture_timer_cb                                      proc_sys_write
__kmem_cache_free                                         proc_sys_call_handler
kmemleak_free                                             drop_caches_sysctl_handler
delete_object_full                                        drop_slab
__delete_object                                           shrink_slab
put_object                                                lazy_rcu_shrink_scan
call_rcu                                                  rcu_nocb_flush_bypass
__call_rcu_commn                                            rcu_nocb_bypass_lock
                                                            raw_spin_trylock(&rdp->nocb_bypass_lock) fail
                                                            atomic_inc(&rdp->nocb_lock_contended);
rcu_nocb_wait_contended                                     WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
 WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended))                                          |
                            |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __|

Reproduce this bug with "echo 3 > /proc/sys/vm/drop_caches".

This commit therefore uses rcu_nocb_try_flush_bypass() instead of
rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan().  If the nocb_bypass
queue is being flushed, then rcu_nocb_try_flush_bypass will return
directly.
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>