all: avoid repeated default interface lookups

On some platforms (notably macOS and iOS) we look up the default
interface to bind outgoing connections to. This is both duplicated
work and results in logspam when the default interface is not available
(i.e. when a phone has no connectivity, we log an error and thus cause
more things that we will try to upload and fail).

Fixed by passing around a netmon.Monitor to more places, so that we can
use its cached interface state.

Fixes #7850
Updates #7621

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/dnscache/dnscache.go

@@ -21,6 +21,7 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/singleflight"
@@ -91,6 +92,11 @@ type Resolver struct {
 	// be added to all log messages printed with this logger.
 	Logf logger.Logf
 
+	// NetMon optionally provides a netmon.Monitor to use to get the current
+	// (cached) network interface.
+	// If nil, the interface will be looked up dynamically.
+	NetMon *netmon.Monitor
+
 	sf singleflight.Group[string, ipRes]
 
 	mu      sync.Mutex