net/dnsfallback: do not attempt lookups of IPs.

Currently if the passed in host is an IP, Lookup still attempts to
resolve it with a dns server. This makes it just return the IP directly.

Updates tailscale/corp#4475

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/dnsfallback/dnsfallback.go

@@ -27,6 +27,10 @@
 )
 
 func Lookup(ctx context.Context, host string) ([]netaddr.IP, error) {
+	if ip, err := netaddr.ParseIP(host); err == nil && !ip.IsZero() {
+		return []netaddr.IP{ip}, nil
+	}
+
 	type nameIP struct {
 		dnsName string
 		ip      netaddr.IP