kernel: powerpc/rtas: Keep MSR[RI] set when calling RTAS

In the Linux kernel, the following vulnerability has been resolved: powerpc/rtas: Keep MSRRI set when calling RTAS RTAS runs in real mode MSRDR and MSRIR unset and in 32-bit big endian mode MSRSF,LE unset. The change in MSR is done in enterrtas in a relatively complex way, since the MSR value cou...