Call rcu_barrier() in the isc_mem_destroy() just once

Call rcu_barrier() in the isc_mem_destroy() just once

The previous work in this area was led by the belief that we might be
calling call_rcu() from within call_rcu() callbacks.  After carefully
checking all the current callback, it became evident that this is not
the case and the problem isn't enough rcu_barrier() calls, but something
entirely else.

Call the rcu_barrier() just once as that's enough and the multiple
rcu_barrier() calls will not hide the real problem anymore, so we can
find it.