hashtable: add hash_for_each_possible_rcu_notrace()

This adds hash_for_each_possible_rcu_notrace() which is basically
a notrace clone of hash_for_each_possible_rcu() which cannot be
used in real mode due to its tracing/debugging capability.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>