irqchip/gic-v4.1: Use local 4_1 ITS to generate VSGI

On multi-node GICv4.1 system, VSGI senders always use one certain 4_1 ITS,
because find_4_1_its() returns the first its_node in the list, regardless of
which node the VSGI sender is on. This brings guest VSGI performance drop
when VM is not running on the same node as this returned ITS.

On a 2-socket environment, each with one ITS and 32 cpu, GICv4.1 enabled,
4U8G guest, 4 vcpu is running on same socket.

  When the VM is on socket0, kvm-unit-tests ipi_hw result is 850ns.
  When the VM is on socket1, it is 750ns.

The reason is that the VSGI sender always uses the last reported ITS (that
on socket1) to inject VSGI. The access from a CPU to a other-socket ITS
will cost 100ns more compared to an access to the local ITS.

Using the local ITS results in a 12% reduction in IPI latency.

Modify find_4_1_its() to return the first per-CPU local_4_1_its, which is
initialized when the VPE table is inherited from the ITS or from another
CPU.  If it fails to find a local 4_1 ITS, it returns any 4_1 ITS like
before.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Nianyao Tang <tangnianyao@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/20250515145359.2795959-1-tangnianyao@huawei.com

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