lift the xfs writepage code into iomap

[Christoph Hellwig]

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.