If this happens, it results in us pessimistically closing more
connections than might be necessary, but is more correct since we won't
"miss" a change to the default route interface and keep trying to send
data over a nonexistent interface, or one that can't reach the internet.
Updates tailscale/corp#19124
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ia0b8b04cb8cdcb0da0155fd08751c9dccba62c1a
375617c5c8