iomap: move all remaining per-folio logic into iomap_writepage_map

iomap: move all remaining per-folio logic into iomap_writepage_map

Move the tracepoint and the iomap check from iomap_do_writepage into
iomap_writepage_map.  This keeps all logic in one places, and leaves
iomap_do_writepage just as the wrapper for the callback conventions of
write_cache_pages, which will go away when that is converted to an
iterator.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231207072710.176093-7-hch@lst.de
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>