ocfs2: split out ocfs2_prepare_orphan_dir() into locking and prep functions

We do this because ocfs2_create_inode_in_orphan() wants to order locking of
the orphan dir with respect to locking of the inode allocator *before*
making any changes to the directory.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Tao Ma <tao.ma@oracle.com>