Patch from sct fixes a long-standing (I did it!) and rather complex
problem with ext3.

The problem is to do with buffers which are continually being dirtied
by an external agent.  I had code in there (for easily-triggerable
livelock avoidance) which steals the buffer from checkpoint mode and
reattaches it to the running transaction.  This violates ext3 ordering
requirements - it can permit journal space to be reclaimed before the
relevant data has really been written out.

Also, we do have to reliably get a lock on the buffer when moving it
between lists and inspecting its internal state.  Otherwise a competing
read from the underlying block device can trigger an assertion failure,
and a competing write to the underlying block device can confuse ext3
journalling state completely.