reflink is a very complicated process, so it can't be integrated
into one transaction. So if the system panic in the operation, we
may leave a unfinished inode in the destication directory.

So we will try to create an inode in orphan_dir first, reflink it
to the src file and then move it to the destication file in the end.
In that way we won't be afraid of any corruption during the reflink.

This patch adds 2 functions for orphan_dir operation:
1. Create a new inode in orphand dir.
2. Move an inode to a target dir.

Note:
fsck.ocfs2 should work for us to remove the unfinished file in the
orphan_dir.

Signed-off-by: Tao Ma <tao.ma@oracle.com>