hash: Refactor overflow page allocation.

hash: Refactor overflow page allocation.

As with commit b0f18cb77f50a54e997d857d592f6a511617f52c, the goal
here is to move all of the related page modifications to a single
section of code, in preparation for adding write-ahead logging.

Amit Kapila, with slight changes by me.  The larger patch series
of which this is a part has been reviewed and tested by Álvaro
Herrera, Ashutosh Sharma, Mark Kirkwood, Jeff Janes, and Jesper
Pedersen, all of whom should also have been credited in the
previous commit message.