x86/msi: Make irq_retrigger() functional for posted MSI

Luigi reported that retriggering a posted MSI interrupt does not work
correctly.

The reason is that the retrigger happens at the vector domain by sending an
IPI to the actual vector on the target CPU. That works correctly exactly
once because the posted MSI interrupt chip does not issue an EOI as that's
only required for the posted MSI notification vector itself.

As a consequence the vector becomes stale in the ISR, which not only
affects this vector but also any lower priority vector in the affected
APIC because the ISR bit is not cleared.

Luigi proposed to set the vector in the remap PIR bitmap and raise the
posted MSI notification vector. That works, but that still does not cure a
related problem:

  If there is ever a stray interrupt on such a vector, then the related
  APIC ISR bit becomes stale due to the lack of EOI as described above.
  Unlikely to happen, but if it happens it's not debuggable at all.

So instead of playing games with the PIR, this can be actually solved
for both cases by:

 1) Keeping track of the posted interrupt vector handler state

 2) Implementing a posted MSI specific irq_ack() callback which checks that
    state. If the posted vector handler is inactive it issues an EOI,
    otherwise it delegates that to the posted handler.

This is correct versus affinity changes and concurrent events on the posted
vector as the actual handler invocation is serialized through the interrupt
descriptor lock.

Fixes: ed1e48ea4370 ("iommu/vt-d: Enable posted mode for device MSIs")
Reported-by: Luigi Rizzo <lrizzo@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Luigi Rizzo <lrizzo@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20251125214631.044440658@linutronix.de
Closes: https://lore.kernel.org/lkml/20251124104836.3685533-1-lrizzo@google.com

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