md: Allow read error in a single drive raid1 to be passed up.
If a raid1 only has a single working device and gets a read error, we choose to simply return that error up to the filesystem (or whatever) rather than failing the whole array. However the codes doesn't quite do that. We attempt a readbalance which allocates the same drive, so we retry the read - indefinitely. Instead: If read_balance in the error case chooses the same drive that just failed, treat it as a failure and don't retry. Signed-off-by: NeilBrown <email@example.com>