MS_ASYNC will currently wait on previously-submitted I/O, then start new I/O
and not wait on it.  This can cause undesirable blocking if msync is called
rapidly against the same memory.

So instead, change msync(MS_ASYNC) to not start any IO at all.  Just flush
the pte dirty bits into the pageframe and leave it at that.

The IO _will_ happen within a kupdate period.  And the application can use
fsync() or fadvise(FADV_DONTNEED) if it actually wants to schedule the IO
immediately.

(This has triggered an ext3 bug - the page's buffers get dirtied so fast
that kjournald keeps writing the buffers over and over for 10-20 seconds
before deciding to give up for some reason)