apparmor: fix incorrect type assignment when freeing proxies

sparse reports

poisoning the proxy->label before freeing the struct is resulting in
a sparse build warning.
../security/apparmor/label.c:52:30: warning: incorrect type in assignment (different address spaces)
../security/apparmor/label.c:52:30:    expected struct aa_label [noderef] <asn:4>*label
../security/apparmor/label.c:52:30:    got struct aa_label *<noident>

fix with RCU_INIT_POINTER as this is one of those cases where
rcu_assign_pointer() is not needed.
Signed-off-by: NJohn Johansen <john.johansen@canonical.com>

security/apparmor/label.c

@@ -49,7 +49,7 @@ static void free_proxy(struct aa_proxy *proxy)
 		/* p->label will not updated any more as p is dead */
 		aa_put_label(rcu_dereference_protected(proxy->label, true));
 		memset(proxy, 0, sizeof(*proxy));
-		proxy->label = (struct aa_label *) PROXY_POISON;
+		RCU_INIT_POINTER(proxy->label, (struct aa_label *)PROXY_POISON);
 		kfree(proxy);
 	}
 }