Currently the restrictions are too harsh: there is a time interval
between an instance gets a new disk and before it is added to the
configuration in which the restriction is not met. We solve this by
allowing temporary DRBD minors to match existing minors (for the same
instance), such that parallel creations/minor allocations are OK.

The change is done by moving the add of temporary minors to the
minor map after the instance minors are computed, and only considering
them as duplicate if the instance name doesn't match.

Reviewed-by: ultrotter