The low level kernel interface to RTAS (the firmware runtime services)
was plagued with races that could cause from bogus results of RTAS
operations to total machine crashes in some circumstances. This patch
fix the ones I could identify, hoping I didn't miss any. I also added
a WARN_ON (well, it's asm equivalent) to enter_rtas to make sure we
never _ever_ try to call that with interrupts enabled.