Revert "[PATCH] mm: micro optimise zone_watermark_ok"

This reverts commit e80ee884.

Pawel Sikora had a boot-time oops due to it - because the sign change
invalidates the following comparisons, since 'free_pages' can be
negative.

The micro-optimization just isn't worth it.
Bisected-by: NPawel Sikora <pluto@agmk.net>
Acked-by: NAndrew Morton <akpm@osdl.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -989,8 +989,7 @@ int zone_watermark_ok(struct zone *z, int order, unsigned long mark,
 		      int classzone_idx, int alloc_flags)
 {
 	/* free_pages my go negative - that's OK */
-	unsigned long min = mark;
-	long free_pages = z->free_pages - (1 << order) + 1;
+	long min = mark, free_pages = z->free_pages - (1 << order) + 1;
 	int o;
 
 	if (alloc_flags & ALLOC_HIGH)