rxrpc: Allow security classes to give more info on server keys

Allow a security class to give more information on an rxrpc_s-type key when
it is viewed in /proc/keys.  This will allow the upcoming RxGK security
class to show the enctype name here.
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/ar-internal.h

@@ -227,6 +227,9 @@ struct rxrpc_security {
 	/* Destroy the payload of a server key */
 	void (*destroy_server_key)(struct key *);
 
+	/* Describe a server key */
+	void (*describe_server_key)(const struct key *, struct seq_file *);
+
 	/* initialise a connection's security */
 	int (*init_connection_security)(struct rxrpc_connection *,
 					struct rxrpc_key_token *);

net/rxrpc/server_key.c

@@ -105,7 +105,11 @@ static void rxrpc_destroy_s(struct key *key)
 
 static void rxrpc_describe_s(const struct key *key, struct seq_file *m)
 {
+	const struct rxrpc_security *sec = key->payload.data[1];
+
 	seq_puts(m, key->description);
+	if (sec && sec->describe_server_key)
+		sec->describe_server_key(key, m);
 }
 
 /*