md/4.4

md updates for 4.4.

Two major components to this update.

1/ the clustered-raid1 support from SUSE is nearly
  complete.  There are a few outstanding issues being
  worked on.  Maybe half a dozen patches will bring
  this to a usable state.

2/ The first stage of journalled-raid5 support from
   Facebook makes an appearance.  With a journal
   device configured (typically NVRAM or SSD), the
   "RAID5 write hole" should be closed - a crash
   during degraded operations cannot result in data
   corruption.

   The next stage will be to use the journal as a
   write-behind cache so that latency can be reduced
   and in some cases throughput increased by
   performing more full-stripe writes.
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