For data=ordered, kjournald at commit time has to write out and wait upon a
long list of buffers.  It does this in a rather awkward way with a single
list.  it causes complexity and long lock hold times, and makes the addition
of rescheduling points quite hard

So what we do instead (based on Chris Mason's suggestion) is to add a new
buffer list (t_locked_list) to the journal.  It contains buffers which have
been placed under I/O.

So as we walk the t_sync_datalist list we move buffers over to t_locked_list
as they are written out.

When t_sync_datalist is empty we may then walk t_locked_list waiting for the
I/O to complete.

As a side-effect this means that we can remove the nasty synchronous wait in
journal_dirty_data which is there to avoid the kjournald livelock which would
otherwise occur when someone is continuously dirtying a buffer.