net/dnscache: do not call LookupIPFallback if the context was canceled.

When the context is canceled, dc.dialOne returns an error from line 345.
This causes the defer on line 312 to try to resolve the host again, which
triggers a dns lookup of "127.0.0.1" from derp.

Updates tailscale/corp#4475

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

net/dnscache/dnscache.go

@@ -312,7 +312,7 @@ func (d *dialer) DialContext(ctx context.Context, network, address string) (retC
 	defer func() {
 		// On failure, consider that our DNS might be wrong and ask the DNS fallback mechanism for
 		// some other IPs to try.
-		if ret == nil || d.dnsCache.LookupIPFallback == nil || dc.dnsWasTrustworthy() {
+		if ret == nil || ctx.Err() != nil || d.dnsCache.LookupIPFallback == nil || dc.dnsWasTrustworthy() {
 			return
 		}
 		ips, err := d.dnsCache.LookupIPFallback(ctx, host)