CVE-2026-93251: ACPI: bus: Introduce acpi_bus_get_primary_device()

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