The rcu_access_pointer() docbook header correctly notes that it may be
used during post-grace-period teardown.  However, it is usually better to
use rcu_dereference_protected() for this purpose.  This commit therefore
calls out this preferred usage.
Reported-by: NMaxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>