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 <hannes@cmpxchg.org>
Cc: Paul Szabo <psz@maths.usyd.edu.au>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -241,9 +241,6 @@ static unsigned long global_dirtyable_memory(void)
 	if (!vm_highmem_is_dirtyable)
 		x -= highmem_dirtyable_memory(x);
 
-	/* Subtract min_free_kbytes */
-	x -= min_t(unsigned long, x, min_free_kbytes >> (PAGE_SHIFT - 10));
-
 	return x + 1;	/* Ensure that we never return 0 */
 }