Set _mapcount PAGE_BUDDY_MAPCOUNT_VALUE to make the page buddy.  Not the
magic number -2.
Signed-off-by: NWang Sheng-Hui <shhuiw@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>