Currently some callsites of sync_rcu_preempt_exp_done() are not called
with the corresponding rcu_node's ->lock held, which could introduces
bugs as per Paul:

o	CPU 0 in sync_rcu_preempt_exp_done() reads ->exp_tasks and
	sees that it is NULL.

o	CPU 1 blocks within an RCU read-side critical section, so
	it enqueues the task and points ->exp_tasks at it and
	clears CPU 1's bit in ->expmask.

o	All other CPUs clear their bits in ->expmask.

o	CPU 0 reads ->expmask, sees that it is zero, so incorrectly
	concludes that all quiescent states have completed, despite
	the fact that ->exp_tasks is non-NULL.

To fix this, sync_rcu_preempt_exp_unlocked() is introduced to replace
lockless callsites of sync_rcu_preempt_exp_done().

Further, a lockdep annotation is added into sync_rcu_preempt_exp_done()
to prevent mis-use in the future.
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>