Fix statistics for space_extents_illustrate.

Currently space_extents_illustrate gets the total number of pages from
space.pages(). However, since since the this method uses
space.each_xdes, which does not include un-initialized extents,
space.pages is typically much larger than actual number of pages
analyzed.

This patch modifies space_extents_illustrate to use the correct number
of total pages in its statistics.