encrypted-keys: fix rcu and sparse messages

Enabling CONFIG_PROVE_RCU and CONFIG_SPARSE_RCU_POINTER resulted in
"suspicious rcu_dereference_check() usage!" and "incompatible types
in comparison expression (different address spaces)" messages.

Access the masterkey directly when holding the rwsem.

Changelog v1:
- Use either rcu_read_lock()/rcu_derefence_key()/rcu_read_unlock()
or remove the unnecessary rcu_derefence() - David Howells

Reported-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>