futex: Implement FUTEX2_NUMA

Extend the futex2 interface to be numa aware.

When FUTEX2_NUMA is specified for a futex, the user value is extended
to two words (of the same size). The first is the user value we all
know, the second one will be the node to place this futex on.

  struct futex_numa_32 {
	u32 val;
	u32 node;
  };

When node is set to ~0, WAIT will set it to the current node_id such
that WAKE knows where to find it. If userspace corrupts the node value
between WAIT and WAKE, the futex will not be found and no wakeup will
happen.

When FUTEX2_NUMA is not set, the node is simply an extension of the
hash, such that traditional futexes are still interleaved over the
nodes.

This is done to avoid having to have a separate !numa hash-table.

[bigeasy: ensure to have at least hashsize of 4 in futex_init(), add
pr_info() for size and allocation information. Cast the naddr math to
void*]

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/20250416162921.513656-17-bigeasy@linutronix.de

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