powerpc/pseries: Fix use after free in remove_phb_dynamic()

In the Linux kernel, the following vulnerability has been resolved:

powerpc/pseries: Fix use after free in remove_phb_dynamic()

In remove_phb_dynamic() we use &phb->io_resource, after we've called
device_unregister(&host_bridge->dev). But the unregister may have freed
phb, because pcibios_free_controller_deferred() is the release function
for the host_bridge.

If there are no outstanding references when we call device_unregister()
then phb will be freed out from under us.

This has gone mainly unnoticed, but with slub_debug and page_poison
enabled it can lead to a crash:

PID: 7574 TASK: c0000000d492cb80 CPU: 13 COMMAND: "drmgr"
#0 [c0000000e4f075a0] crash_kexec at c00000000027d7dc
#1 [c0000000e4f075d0] oops_end at c000000000029608
#2 [c0000000e4f07650] __bad_page_fault at c0000000000904b4
#3 [c0000000e4f076c0] do_bad_slb_fault at c00000000009a5a8
#4 [c0000000e4f076f0] data_access_slb_common_virt at c000000000008b30
Data SLB Access [380] exception frame:
R0: c000000000167250 R1: c0000000e4f07a00 R2: c000000002a46100
R3: c000000002b39ce8 R4: 00000000000000c0 R5: 00000000000000a9
R6: 3894674d000000c0 R7: 0000000000000000 R8: 00000000000000ff
R9: 0000000000000100 R10: 6b6b6b6b6b6b6b6b R11: 0000000000008000
R12: c00000000023da80 R13: c0000009ffd38b00 R14: 0000000000000000
R15: 000000011c87f0f0 R16: 0000000000000006 R17: 0000000000000003
R18: 0000000000000002 R19: 0000000000000004 R2…