lift the xfs writepage code into iomap v3

[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.


Changes since v2:
 - rebased to v5.3-rc1
 - folded in a few changes from the gfs2 enablement series

Changes since v1:
 - rebased to the latest xfs for-next tree
 - keep the preallocated transactions for size updates
 - rename list_pop to list_pop_entry and related cleanups
 - better document the nofs context handling
 - document that the iomap tracepoints are not a stable API