[PATCH v2] rcu: Change return type of rcu_spawn_one_boost_kthread()

[Byungchul Park]

Hello,

I tested if the WARN_ON_ONCE() is fired with my box and it was ok.

Thanks,
Byungchul

Changes from v1
-. WARN_ON_ONCE() on failing to create rcu_boost_kthread.
-. Changed title and commit message a bit.

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