A RAID1 device does not necessarily need a fullsync if the bitmap can be used instead.
Similar to commit d6b212f4b19da5301e6b6eca562e5c7a2a6e8c8d in raid5.c, if a raid1
device can be brought back (i.e. from a transient failure) it shouldn't need a
complete resync. Provided the bitmap is not to old, it will have recorded the areas
of the disk that need recovery.
** I've used 'saved_raid_disk' here similar to RAID5, but it doesn't seem to fit
as well. The positions aren't really as important as they are in RAID5, and
I'm using the 'saved_raid_disk' as more of an indicator that it can use the
bitmap, rather than for any other purpose. Perhaps the meaning is being
overloaded and a different solution should be found?