rcu: provide a static initializer for hlist_nulls_head

Patch series "ucount: Simplify refcounting with rcuref_t".

I noticed that the atomic_dec_and_lock_irqsave() in put_ucounts() loops
sometimes even during boot.  Something like 2-3 iterations but still. 
This series replaces the refcounting with rcuref_t and adds a RCU lookup.

This allows a lockless lookup in alloc_ucounts() if the entry is available
and a cmpxchg()less put of the item.


This patch (of 4):

Provide a static initializer for hlist_nulls_head so that it can be used
in statically defined data structures.

Link: https://lkml.kernel.org/r/20250203150525.456525-1-bigeasy@linutronix.de
Link: https://lkml.kernel.org/r/20250203150525.456525-2-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai jiangshan <jiangshanlai@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mengen Sun <mengensun@tencent.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: YueHong Wu <yuehongwu@tencent.com>
Cc: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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