In the case where the exit node requires SNAT, we would SNAT all traffic not just the
traffic meant to go through the exit node. This was a result of the default route being
added to the routing table which would match basically everything.
In this case, we need to account for all peers in the routing table not just the ones
that require NAT.
Fix and add a test.
Updates tailscale/corp#8020
Signed-off-by: Maisem Ali <maisem@tailscale.com>
3ae7140690