x86: fix preemptable page type handling

x86: fix preemptable page type handling

- retain a page reference when PGT_partial is set on a page (and drop
  it when clearing that flag)
- don't drop a page reference never acquired when freeing the page
  type
  of a page where the allocation of the type got preempted (and never
  completed)
- don't acquire a page reference when allocating the page type of a
  page where freeing the type got preempted (and never completed, and
  hence didn't drop the respective reference)

Signed-off-by: Jan Beulich <jbeulich@novell.com>