This is pdflush's first application!  The writeback of
the unused inodes list by keventd is removed, and a
pdflush thread is dispatched instead.

There is a need for exclusion - to prevent all the
pdflush threads from working against the same request
queue.  This is implemented locally.  And this is a
problem, because other pdflush threads can be dispatched
to writeback other filesystem objects, and they don't
know that there's already a pdflush thread working that
request queue.

So moving the exclusion into the request queue itself
is on my things-to-do-list.  But the code as-is works
OK - under a `dbench 100' load the number of pdflush
instances can grow as high as four or five.  Some fine
tuning is needed...