TAKE - fix overagressive writeout of mmap pages

[Steve Lord <lord@xxxxxxx>]

Sometimes xfs could allocate space for and write out pages 
which it should not have. Showed up as a failure in fsx
when running under heavy memory load.

Date:  Fri Jun  7 20:27:29 PDT 2002
Workarea:  jen.americas.sgi.com:/src/lord/xfs-linux.2.4

The following file(s) were checked into:
  bonnie.engr.sgi.com:/isms/slinx/2.4.x-xfs


Modid:  2.4.x-xfs:slinx:121106a
linux/fs/xfs/pagebuf/page_buf_io.c - 1.42
        - When clustering unbuffered pages for writeout - only look at dirty 
ones