iommu/amd: Stop irq_remapping_select() matching when remapping is disabled

The AMD IOMMU initialisation registers the IRQ remapping domain for
each IOMMU before doing the final sanity check that every I/OAPIC is
covered.

This means that the AMD irq_remapping_select() function gets invoked
even when IRQ remapping has been disabled, eventually leading to a NULL
pointer dereference in alloc_irq_table().

Unfortunately, the IVRS isn't fully parsed early enough that the sanity
check can be done in time to registering the IRQ domain altogether.
Doing that would be nice, but is a larger and more error-prone task. The
simple fix is just for irq_remapping_select() to refuse to report a
match when IRQ remapping has disabled.

Link: https://lore.kernel.org/lkml/ed4be9b4-24ac-7128-c522-7ef359e8185d@gmx.at
Fixes: a1a785b5 ("iommu/amd: Implement select() method on remapping irqdomain")
Reported-by: Johnathan Smithinovic <johnathan.smithinovic@gmx.at>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/04bbe8bca87f81a3cfa93ec4299e53f47e00e5b3.camel@infradead.orgSigned-off-by: Will Deacon <will@kernel.org>

drivers/iommu/amd/iommu.c

@@ -3854,6 +3854,9 @@ static int irq_remapping_select(struct irq_domain *d, struct irq_fwspec *fwspec,
 	struct amd_iommu *iommu;
 	int devid = -1;
 
+	if (!amd_iommu_irq_remap)
+		return 0;
+
 	if (x86_fwspec_is_ioapic(fwspec))
 		devid = get_ioapic_devid(fwspec->param[0]);
 	else if (x86_fwspec_is_hpet(fwspec))