rcu/nocb: Invert rcu_state.barrier_mutex VS hotplug lock locking order

In case of failure to spawn either rcuog or rcuo[p] kthreads for a given
rdp, rcu_nocb_rdp_deoffload() needs to be called with the hotplug
lock and the barrier_mutex held. However cpus write lock is already held
while calling rcutree_prepare_cpu(). It's not possible to call
rcu_nocb_rdp_deoffload() from there with just locking the barrier_mutex
or this would result in a locking inversion against
rcu_nocb_cpu_deoffload() which holds both locks in the reverse order.

Simply solve this with inverting the locking order inside
rcu_nocb_cpu_[de]offload(). This will be a pre-requisite to toggle NOCB
states toward cpusets anyway.

Signed-off-by: Zqiang <[email protected]>
Cc: Neeraj Upadhyay <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Uladzislau Rezki <[email protected]>
Cc: Joel Fernandes <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Neeraj Upadhyay <[email protected]>

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