PCI/PM: Prevent runtime suspend until devices are fully initialized

Previously, it was possible for a PCI device to be runtime-suspended before
it was fully initialized. When that happened, the suspend process could
save invalid device state, for example, before BAR assignment. Restoring
the invalid state during resume may leave the device non-functional.

Prevent runtime suspend for PCI devices until they are fully initialized by
deferring pm_runtime_enable().

More details on how exactly this may occur:

  1. PCI device is created by pci_scan_slot() or similar

  2. As part of pci_scan_slot(), pci_pm_init() puts the device in D0 and
     prevents runtime suspend prevented via pm_runtime_forbid()

  3. pci_device_add() adds the underlying 'struct device' via device_add(),
     which means user space can allow runtime suspend, e.g.,

       echo auto > /sys/bus/pci/devices/.../power/control

  4. PCI device receives BAR configuration
     (pci_assign_unassigned_bus_resources(), etc.)

  5. pci_bus_add_device() applies final fixups, saves device state, and
     tries to attach a driver

The device may potentially be suspended between #3 and #5, so this is racy
with user space (udev or similar).

Many PCI devices are enumerated at subsys_initcall time and so will not
race with user space, but devices created later by hotplug or modular
pwrctrl or host controller drivers are susceptible to this race.

More runtime PM details at the first Link: below.

Link: https://lore.kernel.org/all/0e35a4e1-894a-47c1-9528-fc5ffbafd9e2@samsung.com/
Signed-off-by: Brian Norris <briannorris@chromium.org>
[bhelgaas: update comments per https://lore.kernel.org/r/CAJZ5v0iBNOmMtqfqEbrYyuK2u+2J2+zZ-iQd1FvyCPjdvU2TJg@mail.gmail.com]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260122094815.v5.1.I60a53c170a8596661883bd2b4ef475155c7aa72b@changeid

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