PCI: Make pci_bus_type constant

Since commit d492cc2 ("driver core: device.h: make struct bus_type a
const *"), the driver core can properly handle constant struct bus_type,
move the pci_bus_type variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Link: https://lore.kernel.org/r/20240823074202.139265-1-kunwu.chan@linux.dev
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: KunWuChan

drivers/pci/pci-driver.c

@@ -1670,7 +1670,7 @@ static void pci_dma_cleanup(struct device *dev)
 		iommu_device_unuse_default_domain(dev);
 }
 
-struct bus_type pci_bus_type = {
+const struct bus_type pci_bus_type = {
 	.name		= "pci",
 	.match		= pci_bus_match,
 	.uevent		= pci_uevent,

include/linux/pci.h

@@ -1098,7 +1098,7 @@ enum pcie_bus_config_types {
 
 extern enum pcie_bus_config_types pcie_bus_config;
 
-extern struct bus_type pci_bus_type;
+extern const struct bus_type pci_bus_type;
 
 /* Do NOT directly access these two variables, unless you are arch-specific PCI
  * code, or PCI core code. */