Merge tag 'md-3.4-fixes' of git://neil.brown.name/md

Pull a few more md bug fixes from NeilBrown:
 "2 are tagged for -stable, one being for a fairly serious bug that can
  corrupt metadata and make it hard to recovery an array.  The other is
  for a more recent regression since 3.3"

* tag 'md-3.4-fixes' of git://neil.brown.name/md:
  md: fix possible corruption of array metadata on shutdown.
  md: don't call ->add_disk unless there is good reason.
  DM RAID: Use safe version of rdev_for_each