ocfs2: Set journal descriptor to NULL after journal shutdown

Patch sets journal descriptor to NULL after the journal is shutdown.
This ensures that jbd2_journal_release_jbd_inode(), which removes the
jbd2 inode from txn lists, can be called safely from ocfs2_clear_inode()
even after the journal has been shutdown.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>