When a connector receives a packet from a client on a transit IP that it
can't find a real IP mapping for, it drops the packet. This commit
starts notifying the client of this dropping over TSMP, so the client
can tell the connector to re-establish the transit IP-real IP binding.
Updates tailscale/corp#34256.
Signed-off-by: Naman Sood <mail@nsood.in>