Restructure urcu_updater() to more accurately reflect actual failure scenario

Restructure urcu_updater() to more accurately reflect actual failure
scenario.

This allows an easier transformation to force failure -- simple #ifdef
out the second counter flip out of urcu_updater()'s model of
"current synchronize_rcu()".

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>