timers-urgent-2020-04-12

 - Fix the time_for_children symlink in /proc/$PID/ so it properly reflects
   that it part of the 'time' namespace

 - Add the missing userns limit for the allowed number of time namespaces,
   which was half defined but the actual array member was not added.  This
   went unnoticed as the array has an exessive empty member at the end but
   introduced a user visible regression as the output was corrupted.

 - Prevent further silent ucount corruption by adding a BUILD_BUG_ON() to
   catch half updated data.