Merge branch 'md-next' of git://git.kernel.org/pub/scm/linux/kernel/git/song/md into...

Pull MD changes from Song.

* 'md-next' of git://git.kernel.org/pub/scm/linux/kernel/git/song/md:
  md/raid1: introduce wait_for_serialization
  md/raid1: use bucket based mechanism for IO serialization
  md: introduce a new struct for IO serialization
  md: don't destroy serial_info_pool if serialize_policy is true
  raid1: serialize the overlap write
  md: reorgnize mddev_create/destroy_serial_pool
  md: add serialize_policy sysfs node for raid1
  md: prepare for enable raid1 io serialization
  md: fix a typo s/creat/create
  md: rename wb stuffs
  raid5: remove worker_cnt_per_group argument from alloc_thread_groups
  md/raid6: fix algorithm choice under larger PAGE_SIZE
  raid6/test: fix a compilation warning
  raid6/test: fix a compilation error
  md-bitmap: small cleanups