dax: advance the iomap_iter on zero range

Update the DAX zero range iomap iter handler to advance the iter
directly. Advance by the full length in the hole/unwritten case, or
otherwise advance incrementally in the zeroing loop. In either case,
return 0 or an error code for success or failure.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250224144757.237706-7-bfoster@redhat.com
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Brian Foster

fs/dax.c

@@ -1358,47 +1358,48 @@ static s64 dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
 {
 	const struct iomap *iomap = &iter->iomap;
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
-	loff_t pos = iter->pos;
 	u64 length = iomap_length(iter);
-	s64 written = 0;
+	s64 ret;
 
 	/* already zeroed?  we're done. */
 	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
-		return length;
+		return iomap_iter_advance(iter, &length);
 
 	/*
 	 * invalidate the pages whose sharing state is to be changed
 	 * because of CoW.
 	 */
 	if (iomap->flags & IOMAP_F_SHARED)
 		invalidate_inode_pages2_range(iter->inode->i_mapping,
-					      pos >> PAGE_SHIFT,
-					      (pos + length - 1) >> PAGE_SHIFT);
+				iter->pos >> PAGE_SHIFT,
+				(iter->pos + length - 1) >> PAGE_SHIFT);
 
 	do {
+		loff_t pos = iter->pos;
 		unsigned offset = offset_in_page(pos);
-		unsigned size = min_t(u64, PAGE_SIZE - offset, length);
 		pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
-		long rc;
 		int id;
 
+		length = min_t(u64, PAGE_SIZE - offset, length);
+
 		id = dax_read_lock();
-		if (IS_ALIGNED(pos, PAGE_SIZE) && size == PAGE_SIZE)
-			rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
+		if (IS_ALIGNED(pos, PAGE_SIZE) && length == PAGE_SIZE)
+			ret = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
 		else
-			rc = dax_memzero(iter, pos, size);
+			ret = dax_memzero(iter, pos, length);
 		dax_read_unlock(id);
 
-		if (rc < 0)
-			return rc;
-		pos += size;
-		length -= size;
-		written += size;
+		if (ret < 0)
+			return ret;
+
+		ret = iomap_iter_advance(iter, &length);
+		if (ret)
+			return ret;
 	} while (length > 0);
 
 	if (did_zero)
 		*did_zero = true;
-	return written;
+	return ret;
 }
 
 int dax_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,