dm-raid1 improvements

Seemed odd to me, too.  Another odd thing is that this kernel release, which I had not really
thought was anything all that special, actually has not one, but at least two bombshell
features.  How did I miss the loud outcry which surely occurred when read balance support in
raid1 was lost?  And why have both its return, and the new TASK_KILLABLE functionality gotten
so little notice?