percpu: move common parts out of pcpu_[de]populate_chunk()

percpu-vm and percpu-km implement separate versions of
pcpu_[de]populate_chunk() and some part which is or should be common
are currently in the specific implementations.  Make the following
changes.

* Allocate area clearing is moved from the pcpu_populate_chunk()
  implementations to pcpu_alloc().  This makes percpu-km's version
  noop.

* Quick exit tests in pcpu_[de]populate_chunk() of percpu-vm are moved
  to their respective callers so that they are applied to percpu-km
  too.  This doesn't make any meaningful difference as both functions
  are noop for percpu-km; however, this is more consistent and will
  help implementing atomic allocation support.

Signed-off-by: Tejun Heo <tj@kernel.org>