mm: Don't touch uninitialized variable in do_pages_stat_array()

Commit 80bba1290ab5122c60cdb73332b26d288dc8aedd removed one necessary
variable initialization.  As a result following warning happened:

    CC      mm/migrate.o
  mm/migrate.c: In function 'sys_move_pages':
  mm/migrate.c:1001: warning: 'err' may be used uninitialized in this function

More unfortunately, if find_vma() failed, kernel read uninitialized
memory.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
CC: Brice Goglin <Brice.Goglin@inria.fr>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -998,7 +998,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 		unsigned long addr = (unsigned long)(*pages);
 		struct vm_area_struct *vma;
 		struct page *page;
-		int err;
+		int err = -EFAULT;
 
 		vma = find_vma(mm, addr);
 		if (!vma)