Writeback/pdflush cleanup patch from Steven Augart

* Exposes nr_pdflush_threads as /proc/sys/vm/nr_pdflush_threads, read-only.

  (I like this - I expect that management of the pdflush thread pool
  will be important for many-spindle machines, and this is a neat way
  of getting at the info).

* Adds minimum and maximum checking to the five writable pdflush
  and fs-writeback  parameters.

* Minor indentation fix in sysctl.c

* mm/pdflush.c now includes linux/writeback.h, which prototypes
  pdflush_operation.  This is so that the compiler can
  automatically check that the prototype matches the definition.

* Adds a few comments to existing code.