ocfs2: Plugs race between the dc thread and an unlock ast message

This patch plugs a race between the downconvert thread and an unlock ast message.
Specifically, after the downconvert worker has done its task, the dc thread needs
to check whether an unlock ast made the downconvert moot.

Reported-by: David Teigland <teigland@redhat.com>
Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Acked-by: Mark Fasheh <mfasheh@sus.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>