x86: simplify page reference handling for partially (in-)validated pages

x86: simplify page reference handling for partially (in-)validated pages

Simplify general page reference management for preempted (partially
[in-]validated) pages: Reserve on reference that can be acquired
without the risk of overflowing the reference count, thus allowing to
have a simplified get_page() equivalent that cannot fail (but must be
used with care).

Doing this conversion pointed out a latent issue in the changes done
previously in this area: The extra reference must be acquired before
the 'normal' reference gets dropped, so the patch fixes this at once
in both the alloc_page_type() and free_page_type() paths (it's really
only the latter that failed to work with the change described above).

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