rcu: Fix comment for rcu_dereference_raw_notrace

rcu_dereference_raw() calls indirectly rcu_read_lock_held() while
rcu_dereference_raw_notrace() does not so fix the comment about the latter.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>