ACPI: bus: Introduce acpi_bus_get_primary_device()

The function used for obtaining the first "physical" device for which
the given ACPI one is the ACPI companion, acpi_get_first_physical_node(),
may return a stale device pointer (mostly in theory) because
acpi_unbind_one() may run as a whole after dropping the ACPI device's
physical_node_lock in acpi_get_first_physical_node() and before it
returns.  The last reference to the "physical" device may be dropped
then before the pointer to it is returned to the caller.

If that happens and the acpi_get_first_physical_node() caller invokes
get_device() on the pointer obtained from it, which is done by the
majority of its callers, a use-after-free will occur.

To prepare for addressing this problem, introduce a new function for
getting the first "physical" device associated with the given ACPI one
(the "primary physical device") that will also reference count the
device in question before returning a pointer to it.

Make that new function and acpi_get_first_physical_node() share the
physical node list lookup code.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/2843318.mvXUDI8C0e@rafael.j.wysocki

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