fsdax: use a saner calling convention for copy_cow_page_dax

Just pass the vm_fault and iomap_iter structures, and figure out the rest
locally.  Note that this requires moving dax_iomap_sector up in the file.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20211129102203.2243509-14-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>