PM/Hibernate: Do not release preallocated memory unnecessarily (rev. 2)

Since the hibernation code is now going to use allocations of memory
to make enough room for the image, it can also use the page frames
allocated at this stage as image page frames.  The low-level
hibernation code needs to be rearranged for this purpose, but it
allows us to avoid freeing a great number of pages and allocating
these same pages once again later, so it generally is worth doing.

[rev. 2: Take highmem into account correctly.]

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>