iommu/ipmmu-vmsa: Fix crash on early domain free

If iommu_ops.add_device() fails, iommu_ops.domain_free() is still
called, leading to a crash, as the domain was only partially
initialized:

    ipmmu-vmsa e67b0000.mmu: Cannot accommodate DMA translation for IOMMU page tables
    sata_rcar ee300000.sata: Unable to initialize IPMMU context
    iommu: Failed to add device ee300000.sata to group 0: -22
    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000038
    ...
    Call trace:
     ipmmu_domain_free+0x1c/0xa0
     iommu_group_release+0x48/0x68
     kobject_put+0x74/0xe8
     kobject_del.part.0+0x3c/0x50
     kobject_put+0x60/0xe8
     iommu_group_get_for_dev+0xa8/0x1f0
     ipmmu_add_device+0x1c/0x40
     of_iommu_configure+0x118/0x190

Fix this by checking if the domain's context already exists, before
trying to destroy it.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Fixes: d25a2a16 ('iommu: Add driver for Renesas VMSA-compatible IPMMU')
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/ipmmu-vmsa.c

@@ -498,6 +498,9 @@ static int ipmmu_domain_init_context(struct ipmmu_vmsa_domain *domain)
 
 static void ipmmu_domain_destroy_context(struct ipmmu_vmsa_domain *domain)
 {
+	if (!domain->mmu)
+		return;
+
 	/*
 	 * Disable the context. Flush the TLB as required when modifying the
 	 * context registers.