KEYS: Add an RCU payload dereference macro

Add an RCU payload dereference macro as this seems to be a common piece of code
amongst key types that use RCU referenced payloads.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/security/keys/user_defined.c b/security/keys/user_defined.c
index 02807fb..c6ca866 100644
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -184,8 +184,7 @@
 	struct user_key_payload *upayload;
 	long ret;
 
-	upayload = rcu_dereference_protected(
-		key->payload.data, rwsem_is_locked(&((struct key *)key)->sem));
+	upayload = rcu_dereference_key(key);
 	ret = upayload->datalen;
 
 	/* we can return the data as is */