lift the xfs writepage code into iomap v7

[Christoph Hellwig]

Hi all,

this series cleans up the xfs writepage code and then lifts it to
fs/iomap/ so that it could be use by other file system.

Note that in this version a lot of the reviewed-by tag got dropped
as the patch organization changed quite a bit (the actual final
result changes very little, though).

Changes since v6:
 - actually add trace.c to the patch
 - move back to the old order that massages XFS into shape and then
   lifts the code to iomap
 - cleanup iomap_ioend_compare
 - cleanup the add_to_ioend checks

Changes since v5:
 - move the tracing code to fs/iomap/trace.[ch]
 - fix a bisection issue with the tracing code
 - add an assert that xfs_end_io now only gets "complicated" completions
 - better document the iomap_writeback_ops methods in iomap.h

Changes since v4:
 - rebased on top 5.4-rc1
 - drop the addition of list_pop / list_pop_entry
 - re-split a few patches to better fit Darricks scheme of keeping the
   iomap additions separate from the XFS switchover

Changes since v3:
 - re-split the pages to add new code to iomap and then switch xfs to
   it later (Darrick)

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