iommu/mediatek: Fix crash on isr after kexec()

If the system is rebooted via isr(), the IRQ handler might
be triggered before the domain is initialized. Resulting on
an invalid memory access error.

Fix:
[    0.500930] Unable to handle kernel read from unreadable memory at virtual address 0000000000000070
[    0.501166] Call trace:
[    0.501174]  report_iommu_fault+0x28/0xfc
[    0.501180]  mtk_iommu_isr+0x10c/0x1c0

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20221125-mtk-iommu-v2-0-e168dff7d43e@chromium.org
[ joro: Fixed spelling in commit message ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: ribalda

drivers/iommu/mtk_iommu.c

@@ -462,7 +462,7 @@ static irqreturn_t mtk_iommu_isr(int irq, void *dev_id)
 		fault_larb = data->plat_data->larbid_remap[fault_larb][sub_comm];
 	}
 
-	if (report_iommu_fault(&dom->domain, bank->parent_dev, fault_iova,
+	if (!dom || report_iommu_fault(&dom->domain, bank->parent_dev, fault_iova,
 			       write ? IOMMU_FAULT_WRITE : IOMMU_FAULT_READ)) {
 		dev_err_ratelimited(
 			bank->parent_dev,