pageinspect: Fix hash_bitmap_info not to read the underlying page.

It did that to verify that the page was an overflow page rather than
anything else, but that means that checking the status of all the
overflow bits requires reading the entire index.  So don't do that.
The new code validates that the page is not a primary bucket page
or bitmap page by looking at the metapage, so that using this on
large numbers of pages can be reasonably efficient.

Ashutosh Sharma, per a complaint from me, and with further
modifications by me.