rxrpc: Allow the app to store private data on peer structs

Provide a way for the application (e.g. the afs filesystem) to store
private data on the rxrpc_peer structs for later retrieval via the call
object.

This will allow afs to store a pointer to the afs_server object on the
rxrpc_peer struct, thereby obviating the need for afs to keep lookup tables
by which it can associate an incoming call with server that transmitted it.

Signed-off-by: David Howells <[e-mail]>
cc: Marc Dionne <[e-mail]>
cc: Simon Horman <[e-mail]>
cc: [e-mail]
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/rxrpc/oob.c

···
 	switch (type) {
 	case RXRPC_OOB_CHALLENGE:
 		*_peer		= sp->chall.conn->peer;
-		*_peer_appdata	= 0; /* TODO: retrieve appdata */
+		*_peer_appdata	= sp->chall.conn->peer->app_data;
 		break;
 	default:
 		WARN_ON_ONCE(1);
···
 	struct rxrpc_skb_priv *sp = rxrpc_skb(challenge);
 
 	*_peer		= sp->chall.conn->peer;
-	*_peer_appdata	= 0; /* TODO: retrieve appdata */
+	*_peer_appdata	= sp->chall.conn->peer->app_data;
 	*_service_id	= sp->hdr.serviceId;
 	*_security_index = sp->hdr.securityIndex;
 }