raid1, raid5 and multipath maintain their own
'operational' flag.  This is equivalent to
   !rdev->faulty
and so isn't needed.
Similarly raid1 and raid1 maintain a "write_only" flag
that is equivalnt to
   !rdev->in_sync
so it isn't needed either.

As part of implementing this change, we introduce some extra
flag bit in raid5 that are meaningful only inside 'handle_stripe'.
Some of these replace the "action" array which recorded what
actions were required (and would be performed after the stripe
spinlock was released).  This has the advantage of reducing our
dependance on MD_SB_DISKS which personalities shouldn't need
to know about.