PCI: Fix driver_managed_dma check

Since it's not currently safe to take device_lock() in the IOMMU probe
path, that can race against really_probe() setting dev->driver before
attempting to bind. The race itself isn't so bad, since we're only
concerned with dereferencing dev->driver itself anyway, but sadly my
attempt to implement the check with minimal churn leads to a kind of
Time-of-Check to Time-of-Use (TOCTOU) issue, where dev->driver becomes
valid after to_pci_driver(NULL) is already computed, and thus the check
fails to work as intended.

Will and I both hit this with the platform bus, but the pattern here is
the same, so fix it for correctness too.

Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Reported-by: Will McVicker <[e-mail]>
Signed-off-by: Robin Murphy <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Reviewed-by: Will McVicker <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

drivers/pci/pci-driver.c

···
  */
 static int pci_dma_configure(struct device *dev)
 {
-	struct pci_driver *driver = to_pci_driver(dev->driver);
+	const struct device_driver *drv = READ_ONCE(dev->driver);
 	struct device *bridge;
 	int ret = 0;
 
···
 
 	pci_put_host_bridge_device(bridge);
 
-	/* @driver may not be valid when we're called from the IOMMU layer */
-	if (!ret && dev->driver && !driver->driver_managed_dma) {
+	/* @drv may not be valid when we're called from the IOMMU layer */
+	if (!ret && drv && !to_pci_driver(drv)->driver_managed_dma) {
 		ret = iommu_device_use_default_domain(dev);
 		if (ret)
 			arch_teardown_dma_ops(dev);