Tetsuo reports that free page statistics are not reporting correctly,
and Vlastimil noticed that "mm: Move buddy list manipulations into
helpers" botched one of its conversions of add_to_free_area(). Fix the
double-increment of ->nr_free.

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Reported-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>