keys: Network namespace domain tag

Create key domain tags for network namespaces and make it possible to
automatically tag keys that are used by networked services (e.g. AF_RXRPC,
AFS, DNS) with the default network namespace if not set by the caller.

This allows keys with the same description but in different namespaces to
coexist within a keyring.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: netdev@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-afs@lists.infradead.org

include/net/net_namespace.h

@@ -71,6 +71,9 @@ struct net {
 						 */
 	struct llist_node	cleanup_list;	/* namespaces on death row */
 
+#ifdef CONFIG_KEYS
+	struct key_tag		*key_domain;	/* Key domain of operation tag */
+#endif
 	struct user_namespace   *user_ns;	/* Owning user namespace */
 	struct ucounts		*ucounts;
 	spinlock_t		nsid_lock;