pg_buffercache: Change page_num type to bigint

pg_buffercache: Change page_num type to bigint

The page_num was defined as integer, which should be sufficient for the
near future (with 4K pages it's 8TB). But it's virtually free to return
bigint, and get a wider range. This was agreed on the thread, but I
forgot to tweak this in ba2a3c2302f1.

While at it, make the data types in CREATE VIEW a bit more consistent.

Discussion: https://postgr.es/m/CAKZiRmxh6KWo0aqRqvmcoaX2jUxZYb4kGp3N%3Dq1w%2BDiH-696Xw%40mail.gmail.co