ocfs2: Use proper parameter for some inode operation.

In order to make the original function more suitable for reflink,
we modify the following inode operations. Both are tiny.

1. ocfs2_mknod_locked only use dentry for mlog, so move it to
   the caller so that reflink can use it without dentry.
2. ocfs2_prepare_orphan_dir only want inode to get its ip_blkno.
   So use ip_blkno instead.

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