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]