PCI: Allow a NULL "parent" pointer in pci_bus_assign_domain_nr()

pci_create_root_bus() passes a "parent" pointer to
pci_bus_assign_domain_nr().  When CONFIG_PCI_DOMAINS_GENERIC is defined,
pci_bus_assign_domain_nr() dereferences that pointer.  Many callers of
pci_create_root_bus() supply a NULL "parent" pointer, which leads to a NULL
pointer dereference error.

7c67470009 ("PCI: Move domain assignment from arm64 to generic code")
moved the "parent" dereference from arm64 to generic code.  Only arm64 used
that code (because only arm64 defined CONFIG_PCI_DOMAINS_GENERIC), and it
always supplied a valid "parent" pointer.  Other arches supplied NULL
"parent" pointers but didn't defined CONFIG_PCI_DOMAINS_GENERIC, so they
used a no-op version of pci_bus_assign_domain_nr().

8c7d14746a ("ARM/PCI: Move to generic PCI domains") defined
CONFIG_PCI_DOMAINS_GENERIC on ARM, and many ARM platforms use
pci_common_init(), which supplies a NULL "parent" pointer.
These platforms (cns3xxx, dove, footbridge, iop13xx, etc.) crash
with a NULL pointer dereference like this while probing PCI:

  Unable to handle kernel NULL pointer dereference at virtual address 000000a4
  PC is at pci_bus_assign_domain_nr+0x10/0x84
  LR is at pci_create_root_bus+0x48/0x2e4
  Kernel panic - not syncing: Attempted to kill init!

[bhelgaas: changelog, add "Reported:" and "Fixes:" tags]
Reported: http://forum.doozan.com/read.php?2,17868,22070,quote=1
Fixes: 8c7d14746a ("ARM/PCI: Move to generic PCI domains")
Fixes: 7c67470009 ("PCI: Move domain assignment from arm64 to generic code")
Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
CC: stable@vger.kernel.org	# v4.0+

drivers/pci/pci.c

@@ -4772,8 +4772,10 @@ int pci_get_new_domain_nr(void)
 void pci_bus_assign_domain_nr(struct pci_bus *bus, struct device *parent)
 {
 	static int use_dt_domains = -1;
-	int domain = of_get_pci_domain_nr(parent->of_node);
+	int domain = -1;
 
+	if (parent)
+		domain = of_get_pci_domain_nr(parent->of_node);
 	/*
 	 * Check DT domain and use_dt_domains values.
 	 *