PM: hibernate: don't store zero pages in the image file

On ChromeOS we've observed a considerable number of in-use pages filled with
zeros. Today with hibernate it's entirely possible that saveable pages are just
zero filled. Since we're already copying pages word-by-word in do_copy_page it
becomes almost free to determine if a page was completely filled with zeros.

This change introduces a new bitmap which will track these zero pages. If a page
is zero it will not be included in the saved image, instead to track these zero
pages in the image file we will introduce a new flag which we will set on the
packed PFN list. When reading back in the image file we will detect these zero
page PFNs and rebuild the zero page bitmap.

When the image is being loaded through calls to write_next_page if we encounter
a zero page we will silently memset it to 0 and then continue on to the next
page. Given the implementation in snapshot_read_next/snapshot_write_next this
change  will be transparent to non-compressed/compressed and swsusp modes of
operation.

To provide some concrete numbers from simple ad-hoc testing, on a device which
was lightly in use we saw that:

PM: hibernation: Image created (964408 pages copied, 548304 zero pages)

Of the approximately 6.2GB of saveable pages 2.2GB (36%) were just zero filled
and could be tracked entirely within the packed PFN list. The savings would
obviously be much lower for lzo compressed images, but even in the case of
compression not copying pages across to the compression threads will still
speed things up. It's also possible that we would see better overall compression
ratios as larger regions of "real data" would improve the compressibility.

Finally, such an approach could dramatically improve swsusp performance
as each one of those zero pages requires a write syscall to reload, by
handling it as part of the packed PFN list we're able to fully avoid
that.
Signed-off-by: Brian Geffon <bgeffon@google.com>
[ rjw: Whitespace adjustments, removal of redundant parentheses ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>