[AF_RXRPC]: Key facility changes for AF_RXRPC

Export the keyring key type definition and document its availability.

Add alternative types into the key's type_data union to make it more useful.
Not all users necessarily want to use it as a list_head (AF_RXRPC doesn't, for
example), so make it clear that it can be used in other ways.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

security/keys/keyring.c

@@ -66,6 +66,8 @@ struct key_type key_type_keyring = {
 	.read		= keyring_read,
 };
 
+EXPORT_SYMBOL(key_type_keyring);
+
 /*
  * semaphore to serialise link/link calls to prevent two link calls in parallel
  * introducing a cycle