Fix a bug where, for a subnet router that advertizes
4via6 route, all packets with a source IP matching
the 4via6 address were being sent to the host itself.
Instead, only send to host packets whose destination
address is host's local address.
Fixestailscale/tailscale#12448
Signed-off-by: Irbe Krumina <irbe@tailscale.com>
Co-authored-by: Andrew Dunham <andrew@du.nham.ca>