From: Rusty Russell <rusty@rustcorp.com.au>

Matthieu Castet <castet.matthieu@free.fr> pointed out that testing
cpu_online(cpu) on a UP system goes BUG().

That's because you're never supposed to ask cpu_online() about a CPU which
is >= NR_CPUS.  msr and cpuid devices use the minor to indicate the CPU
number.  Oops.

Fix is to explicitly test cpu < NR_CPUS.  Using cpu_online() is OK;
although the CPU might go down before you actually read the file, that will
simply cause junk to be returned.