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.