ucount: use RCU for ucounts lookups

The ucounts element is looked up under ucounts_lock.  This can be
optimized by using RCU for a lockless lookup and return and element if the
reference can be obtained.

Replace hlist_head with hlist_nulls_head which is RCU compatible.  Let
find_ucounts() search for the required item within a RCU section and
return the item if a reference could be obtained.  This means
alloc_ucounts() will always return an element (unless the memory
allocation failed).  Let put_ucounts() RCU free the element if the
reference counter dropped to zero.

Link: https://lkml.kernel.org/r/20250203150525.456525-4-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: 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: Steven Rostedt <rostedt@goodmis.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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