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 <neilb@suse.de>