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

Commit 80bba129 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>