Missing devices should be recorded and cause array to operate in degraded mode.
When specifying the devices that compose a DM RAID array, it is possible to denote
failed or missing devices with '-'s. When this occurs, we must set mddev->degraded.
Otherwise, if the missing/failed device comes back, the bitmap will not have
recorded what areas of the array need to be recovered - the array will be assumed to
be in-sync! Additionally, we must mark in the superblock which device was specified
as missing/failed. We do this by setting the appropriate bit in the 'failed_devices'
field. Finally, we must also ensure that the superblock is properly recorded by
setting 'MD_CHANGE_DEVS' in raid_resume. If we do not cause the superblock to be
rewritten by the resume function, it is possible for a stale superblock to be
written by an out-going in-active table (during 'raid_dtr').
Signed-off-by: Jonathan Brassow <email@example.com>