iomap: move the io_folios field out of struct iomap_ioend

The io_folios member in struct iomap_ioend counts the number of folios
added to an ioend.  It is only used at submission time and can thus be
moved to iomap_writepage_ctx instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231207072710.176093-4-hch@lst.de


Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>