lift the xfs writepage code into iomap

From: Christoph Hellwig
Date: Mon Jun 24 2019 - 01:52:59 EST

Hi all,

this series cleans up the xfs writepage code and then lifts it to
fs/iomap.c so that it could be use by other file system. I've been
wanting to this for a while so that I could eventually convert gfs2
over to it, but I never got to it. Now Damien has a new zonefs
file system for semi-raw access to zoned block devices that would
like to use the iomap code instead of reinventing it, so I finally
had to do the work.