futex: Create private_hash() get/put class

This gets us:

  fph = futex_private_hash(key) /* gets fph and inc users */
  futex_private_hash_get(fph)   /* inc users */
  futex_private_hash_put(fph)   /* dec users */

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-8-bigeasy@linutronix.de