Do this to avoid a theoretical (I haven't seen this in practice) race where
the downconvert thread might drop the dentry lock, allowing a remote unlink
to proceed before dropping the inode locks. This could bounce access to the
orphan dir between nodes.

There doesn't seem to be a need to do the same in ocfs2_dentry_iput() as
that's never called for the last ref drop from the downconvert thread.
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>