ext4: use iomap for zeroing blocks in DAX mode

Use iomap infrastructure for zeroing blocks when in DAX mode.
ext4_iomap_begin() handles read requests just fine and that's all that
is needed for iomap_zero_range().

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -3859,8 +3859,10 @@ static int ext4_block_zero_page_range(handle_t *handle,
 	if (length > max || length < 0)
 		length = max;
 
-	if (IS_DAX(inode))
-		return dax_zero_page_range(inode, from, length, ext4_get_block);
+	if (IS_DAX(inode)) {
+		return iomap_zero_range(inode, from, length, NULL,
+					&ext4_iomap_ops);
+	}
 	return __ext4_block_zero_page_range(handle, mapping, from, length);
 }