page_zip_decompress(): Move the function body to page_zip_decompress_low(),
add the parameter do_validate() to enable page_validate() assertion.  Wrap
page_zip_decompress_low() with do_validate=TRUE.

page_zip_validate(): Invoke page_zip_decompress_low() with do_validate=FALSE,
as page_validate() may fail when the compressed page is being updated
in-place.

page_dir_slot_check(): Replace buf_frame_align() with ut_align_down()
in order to avoid an assertion failure in page_zip_validate(), which
will now invoke page_validate() via page_zip_decompress().