Calling emergency_restart directly from the softirq/bh ipod icmp handler
has a history of causing panics prior to the intended shutdown on both
aarch64 and powerpc64.  The panics didn't inhibit the intended reboot,
but on OPAL-booted ppc64le, were very noisy, because each "hyperthread"
cpu attempted a dealloc of the irq handler from within the handler, so
the console got a noisy stack trace for each thread.

So now if IPOD_QUEUE_RESTART is defined (and it is by default on aarch64
and powerpc64), we move the call to emergency_restart into a dedicated,
preallocated workqueue thread, whose sole purpose is to (eventually)
reboot the machine.  Wasteful, but we don't want to need to initialize
anything, or use a shared workqueue, if an IPOD is really necessary, at
IPOD time.