Revert "PCI: Remove struct pci_dev->driver"

This reverts commit b5f9c644eb.

Revert b5f9c644eb ("PCI: Remove struct pci_dev->driver"), which is needed
to revert 2a4d9408c9 ("PCI: Use to_pci_driver() instead of
pci_dev->driver").

2a4d9408c9 caused a NULL pointer dereference reported by Robert Święcki.
Details in the revert of that commit.

Fixes: 2a4d9408c9 ("PCI: Use to_pci_driver() instead of pci_dev->driver")
Link: https://lore.kernel.org/linux-i2c/CAP145pgdrdiMAT7=-iB1DMgA7t_bMqTcJL4N0=6u8kNY3EU0dw@mail.gmail.com/
Reported-by: Robert Święcki <robert@swiecki.net>
Tested-by: Robert Święcki <robert@swiecki.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci-driver.c

@@ -319,10 +319,12 @@ static long local_pci_probe(void *_ddi)
 	 * its remove routine.
 	 */
 	pm_runtime_get_sync(dev);
+	pci_dev->driver = pci_drv;
 	rc = pci_drv->probe(pci_dev, ddi->id);
 	if (!rc)
 		return rc;
 	if (rc < 0) {
+		pci_dev->driver = NULL;
 		pm_runtime_put_sync(dev);
 		return rc;
 	}
@@ -388,6 +390,7 @@ static int pci_call_probe(struct pci_driver *drv, struct pci_dev *dev,
  * @pci_dev: PCI device being probed
  *
  * returns 0 on success, else error.
+ * side-effect: pci_dev->driver is set to drv when drv claims pci_dev.
  */
 static int __pci_device_probe(struct pci_driver *drv, struct pci_dev *pci_dev)
 {
@@ -462,6 +465,7 @@ static void pci_device_remove(struct device *dev)
 		pm_runtime_put_noidle(dev);
 	}
 	pcibios_free_irq(pci_dev);
+	pci_dev->driver = NULL;
 	pci_iov_remove(pci_dev);
 
 	/* Undo the runtime PM settings in local_pci_probe() */

include/linux/pci.h

@@ -342,6 +342,7 @@ struct pci_dev {
 	u16		pcie_flags_reg;	/* Cached PCIe Capabilities Register */
 	unsigned long	*dma_alias_mask;/* Mask of enabled devfn aliases */
 
+	struct pci_driver *driver;	/* Driver bound to this device */
 	u64		dma_mask;	/* Mask of the bits of bus address this
 					   device implements.  Normally this is
 					   0xffffffff.  You only need to change