Like we do for the ones on iOS.
As a bonus, this removes a caller of tsaddr.IsTailscaleIP which we
want to revamp/remove soonish.
Updates #13687
Change-Id: Iab576a0c48e9005c7844ab52a0aba5ba343b750e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>