x86: prepare setup_pcpu_lpage() for pageattr fix

Make the following changes in preparation of coming pageattr updates.

* Define and use array of struct pcpul_ent instead of array of
  pointers.  The only difference is ->cpu field which is set but
  unused yet.

* Rename variables according to the above change.

* Rename local variable vm to pcpul_vm and move it out of the
  function.

[ Impact: no functional difference ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jan Beulich <JBeulich@novell.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>