[X.25]: Add call forwarding

Adds call forwarding to X.25, allowing it to operate like an X.25 router.
Useful if one needs to manipulate X.25 traffic with tools like tc.
This is an update/cleanup based off a patch submitted by Daniel Ferenci a few years ago.

Thanks Alan for the feedback.
Added the null check to the clones.
Moved the skb_clone's into the forwarding functions.

Worked ok with Cisco XoT, linux X.25 back to back, and some old NTUs/PADs.

Signed-off-by: Andrew Hendry <andrew.hendry@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/x25/x25_route.c b/net/x25/x25_route.c
index 2a3fe98..883a848 100644
--- a/net/x25/x25_route.c
+++ b/net/x25/x25_route.c
@@ -119,6 +119,9 @@
 			__x25_remove_route(rt);
 	}
 	write_unlock_bh(&x25_route_list_lock);
+
+	/* Remove any related forwarding */
+	x25_clear_forward_by_dev(dev);
 }
 
 /*