keys: fix trusted/encrypted keys sparse rcu_assign_pointer messages

Define rcu_assign_keypointer(), which uses the key payload.rcudata instead
of payload.data, to resolve the CONFIG_SPARSE_RCU_POINTER message:
"incompatible types in comparison expression (different address spaces)"

Replace the rcu_assign_pointer() calls in encrypted/trusted keys with
rcu_assign_keypointer().

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>

include/linux/key.h

@@ -293,6 +293,9 @@ static inline bool key_is_instantiated(const struct key *key)
 	(rcu_dereference_protected((KEY)->payload.rcudata,		\
 				   rwsem_is_locked(&((struct key *)(KEY))->sem)))
 
+#define rcu_assign_keypointer(KEY, PAYLOAD)				\
+	(rcu_assign_pointer((KEY)->payload.rcudata, PAYLOAD))
+
 #ifdef CONFIG_SYSCTL
 extern ctl_table key_sysctls[];
 #endif