With data=ordered it is often the case that a quick write-and-truncate will
leave large numbers of pages on the page LRU with no ->mapping, and attached
buffers.  Because ext3 was not ready to let the pages go at the time of
truncation.

These pages are trivially reclaimable, but their seeming absence makes the VM
overcommit accounting confused (they don't count as "free", nor as
pagecache).  And they make the /proc/meminfo stats look odd.

So what we do here is to try to strip the buffers from these pages as the
buffers exit the journal commit.