powerpc/rtas: avoid device tree lookups in rtas_os_term()

In the Linux kernel, the following vulnerability has been resolved: powerpc/rtas: avoid device tree lookups in rtas_os_term() rtas_os_term() is called during panic. Its behavior depends on a couple of conditions in the /rtas node of the device tree, the traversal of which entails locking and local IRQ state changes. If the kernel panics while devtree_lock is held, rtas_os_term() as currently written could hang. Instead of discovering the relevant characteristics at panic time, cache them in file-static variables at boot. Note the lookup for "ibm,extended-os-term" is converted to of_property_read_bool() since it is a boolean property, not an RTAS function token. [mpe: Incorporate suggested change from Nick]