DSR computation fix for fostered nodes

* Fixed DSR computation code to ensure that DSR on fostered content
  is always valid (this bug got triggered in some scenarios with
  changes to p-wrapping and led to content duplication in selser
  mode).

  Extracted from the p-wrapping patch.

Change-Id: Ie92944ab10983c9ceee07257d131c11fcd849d6b