ipn/ipnlocal: add back a watchdog after earlier removal from engine

Commit 2b338dd6a8 removed watchdogEngine because it was weird
(so many methods) and increasingly unnecessary after we'd cleaned up
and simplified so much of the locking.

This adds back a watchdog, but an easier to maintain one that's more
idiomatic.

Updates #19759

Change-Id: I86c458473e126c0809f37696446ce7acf4cc4eb9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/userspace.go

@@ -1541,6 +1541,17 @@ func (e *userspaceEngine) PeerForIP(ip netip.Addr) (ret PeerForIP, ok bool) {
 	e.mu.Lock()
 	nm := e.netMap
 	e.mu.Unlock()
+
+	if !ip.IsValid() {
+		// Treat invalid IPs as just a mutex probe to detect deadlocks.
+		// TODO(bradfitz): extend the Engine interface to have an explicit method for
+		// this purpose, instead of overloading PeerForIP with this special case.
+		// But I'd rather do that at the beginning of a dev cycle.
+		e.wgLock.Lock()
+		defer e.wgLock.Unlock()
+		return ret, false
+	}
+
 	if nm == nil {
 		return ret, false
 	}