mm: fix return value of scan_lru_pages in memory unplug

scan_lru_pages returns pfn. So, it's type should be "unsigned long"
not "int".

Note: I guess this has been work until now because memory hotplug tester's
      machine has not very big memory....
      physical address < 32bit << PAGE_SHIFT.

Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>