[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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/keys.txt

···
 	void unregister_key_type(struct key_type *type);
 
 
+Under some circumstances, it may be desirable to desirable to deal with a
+bundle of keys.  The facility provides access to the keyring type for managing
+such a bundle:
+
+	struct key_type key_type_keyring;
+
+This can be used with a function such as request_key() to find a specific
+keyring in a process's keyrings.  A keyring thus found can then be searched
+with keyring_search().  Note that it is not possible to use request_key() to
+search a specific keyring, so using keyrings in this way is of limited utility.
+
+
 ===================================
 NOTES ON ACCESSING PAYLOAD CONTENTS
 ===================================

include/linux/key.h

···
 	 */
 	union {
 		struct list_head	link;
+		unsigned long		x[2];
+		void			*p[2];
 	} type_data;
 
 	/* key data

security/keys/keyring.c

···
 	.read		= keyring_read,
 };
 
+EXPORT_SYMBOL(key_type_keyring);
+
 /*
  * semaphore to serialise link/link calls to prevent two link calls in parallel
  * introducing a cycle