Revised Ext4 Punch Hole patches

Hi All!

This is my second pass at the Ext4 punch hole support patch set. I've integrated everyone's feedback, and there have been a lot of changes since the last set. Also, I understand that others are also working on routines similar to the helper routines in patch 1. Once we get all our common code worked out, these routines will likely be replaced by the general purpose routines that we come up with later. Thx all for your feed back.

Allison Henderson