- the caller has gotten the inode object, needn't pass the file object.
  And if so, we needn't define a inode pointer variant.
- the position should be aligned by the page size not sector size, so
  we also needn't pass the root object into prepare_pages().
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>