mm: revert "page-writeback.c: subtract min_free_kbytes from dirtyable memory"

mm: revert "page-writeback.c: subtract min_free_kbytes from dirtyable memory"

This reverts commit 75f7ad8e043d.  It was the result of a problem
observed with a 3.2 kernel and merged in 3.9, while the issue had been
resolved upstream in 3.3 (commit ab8fabd46f81: "mm: exclude reserved
pages from dirtyable memory").

The "reserved pages" are a superset of min_free_kbytes, thus this change
is redundant and confusing.  Revert it.

Signed-off-by: Johannes Weiner <[email protected]>
Cc: Paul Szabo <[email protected]>
Cc: Rik van Riel <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>