notifier: Initialize new struct srcu_usage field

notifier: Initialize new struct srcu_usage field

In commit 95433f726301 ("srcu: Begin offloading srcu_struct fields to
srcu_update"), a new struct srcu_usage field was added, but was not
properly initialized. This led to a "spinlock bad magic" BUG when the
SRCU notifier was ever used. This was observed in the MediaTek CCI
devfreq driver on next-20230525. The trimmed stack trace is as follows:

    BUG: spinlock bad magic on CPU#4, swapper/0/1
     lock: 0xffffff80ff529ac0, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
    Call trace:
     spin_bug+0xa4/0xe8
     do_raw_spin_lock+0xec/0x120
     _raw_spin_lock_irqsave+0x78/0xb8
     synchronize_srcu+0x3c/0x168
     srcu_notifier_chain_unregister+0x5c/0xa0
     cpufreq_unregister_notifier+0x94/0xe0
     devfreq_passive_event_handler+0x7c/0x3e0
     devfreq_remove_device+0x48/0xe8

Add __SRCU_USAGE_INIT() to SRCU_NOTIFIER_INIT() so that srcu_usage gets
initialized properly.

Reported-by: Jon Hunter <jonathanh@nvidia.com>
Fixes: 95433f726301 ("srcu: Begin offloading srcu_struct fields to srcu_update")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Sachin Sant <sachinp@linux.ibm.com>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>