rcu: Avoid misordering in nocb_leader_wait()

The NOCB follower wakeup ordering depends on the store to the tail
pointer happening before the wakeup.  However, because atomic_long_add()
does not return a value, it does not provide ordering guarantees, and
the locking in wake_up() only guarantees that the store will happen
before the unlock, which might be too late.  Even though this is only a
theoretical issue, this commit adds a smp_mb__after_atomic() after the
final atomic_long_add() to provide the needed ordering guarantee.

Reported-by: Amit Shah <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Tested-by: Paul Gortmaker <[email protected]>

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