[9.20] chg: dev: 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.

Backport of MR !9134

Merge branch 'backport-4607-call-a-single-rcu_barrier-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9247