From: Anton Blanchard <anton@samba.org>

setup_cpu was being called for the boot cpu after all other cpus had been
spun up.  The init thread can sleep during secondary cpu spinup (eg migration
thread init) and sometimes the init thread would switch to another cpu at
this time.  The end result was setup_cpu was called twice on one cpu and
never on the boot cpu.

The original fix called setup_cpu on the boot cpu before all others but that
wont work for G5, so we now use cpu affinity calls to enforce it.