irqdomain: Add the missing assignment of domain->fwnode for named fwnode

Recently device pass-through stops working for Linux VM running on Hyper-V.

git-bisect shows the regression is caused by the recent commit
467a3bb9 ("PCI: hv: Allocate a named fwnode ..."), but the root cause
is that the commit d59f6617 forgets to set the domain->fwnode for
IRQCHIP_FWNODE_NAMED*, and as a result:

1. The domain->fwnode remains to be NULL.

2. irq_find_matching_fwspec() returns NULL since "h->fwnode == fwnode" is
false, and pci_set_bus_msi_domain() sets the Hyper-V PCI root bus's
msi_domain to NULL.

3. When the device is added onto the root bus, the device's dev->msi_domain
is set to NULL in pci_set_msi_domain().

4. When a device driver tries to enable MSI-X, pci_msi_setup_msi_irqs()
calls arch_setup_msi_irqs(), which uses the native MSI chip (i.e.
arch/x86/kernel/apic/msi.c: pci_msi_controller) to set up the irqs, but
actually pci_msi_setup_msi_irqs() is supposed to call
msi_domain_alloc_irqs() with the hbus->irq_domain, which is created in
hv_pcie_init_irq_domain() and is associated with the Hyper-V chip
hv_msi_irq_chip. Consequently, the irq line is not properly set up, and
the device driver can not receive any interrupt.

Fixes: d59f6617 ("genirq: Allow fwnode to carry name information only")
Fixes: 467a3bb9 ("PCI: hv: Allocate a named fwnode instead of an address-based one")
Reported-by: Lili Deng <v-lide@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/PU1P153MB01694D9AF625AC335C600C5FBFBE0@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM

kernel/irq/irqdomain.c

@@ -149,6 +149,7 @@ struct irq_domain *__irq_domain_add(struct fwnode_handle *fwnode, int size,
 		switch (fwid->type) {
 		case IRQCHIP_FWNODE_NAMED:
 		case IRQCHIP_FWNODE_NAMED_ID:
+			domain->fwnode = fwnode;
 			domain->name = kstrdup(fwid->name, GFP_KERNEL);
 			if (!domain->name) {
 				kfree(domain);