Don't spin in pause() during early boot for kthreads other than thread0.

pause() uses a spin loop to simulate a sleep during early boot. However,
we only need this for thread0 to get far enough in the boot process to
enable timers (at which point pause() can sleep). For other kthreads,
sleeping in pause() is ok as the callout will be scheduled and will
eventually fire once thread0 initializes timers.

Tested by: Steven Kargl
Sleuthing by: markj
MFC after: 1 week
Sponsored by: Netflix