powerpc/rtas: Prevent Spectre v1 gadget construction in sys_rtas()

In the Linux kernel, the following vulnerability has been resolved: powerpc/rtas: Prevent Spectre v1 gadget construction in sys_rtas() Smatch warns: arch/powerpc/kernel/rtas.c:1932 __do_sys_rtas() warn: potential spectre issue 'args.args' [r] (local cap) The 'nargs' and 'nret' locals come directly from a user-supplied buffer and are used as indexes into a small stack-based array and as inputs to copy_to_user() after they are subject to bounds checks. Use array_index_nospec() after the bounds checks to clamp these values for speculative execution.