Reset dial plan when switching profile (#11933)

When switching profile, the server URL can change (e.g.
because of switching to a self-hosted headscale instance).

If it is not reset here, dial plans returned by old
server (e.g. tailscale control server) will be used to
connect to new server (e.g. self-hosted headscale server),
and the register request will be blocked by it until
timeout, leading to very slow profile switches.

Updates #11938 11938

Signed-off-by: Shaw Drastin <showier.drastic0a@icloud.com>

ipn/ipnlocal/local.go

@@ -5881,6 +5881,11 @@ func (b *LocalBackend) SwitchProfile(profile ipn.ProfileID) error {
 	if err := b.pm.SwitchProfile(profile); err != nil {
 		return err
 	}
+
+	// TODO: Check if serverUrl in the switched profile is actually changed;
+	// if not, then no need to reset dial plan
+	b.dialPlan.Store(nil)
+
 	return b.resetForProfileChangeLockedOnEntry(unlock)
 }