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/trusted.c b/security/keys/trusted.c
index 83fc92e..c99b936 100644
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1076,8 +1076,7 @@
 	char *bufp;
 	int i;
 
-	p = rcu_dereference_protected(key->payload.data,
-			rwsem_is_locked(&((struct key *)key)->sem));
+	p = rcu_dereference_key(key);
 	if (!p)
 		return -EINVAL;
 	if (!buffer || buflen <= 0)