net/dns, ipn/ipnlocal: fix regressions from change moving away from deephash

I got sidetracked apparently and never finished writing this Clone
code in 316afe7d02 (#17448). (It really should use views instead.)

And then I missed one of the users of "routerChanged" that was broken up
into "routerChanged" vs "dnsChanged".

This broke integration tests elsewhere.

Fixes #17506

Change-Id: I533bf0fcf3da9ac6eb4a6cdef03b8df2c1fb4c8e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/checkchange/checkchange.go

@@ -17,7 +17,7 @@ type EqualCloner[T any] interface {
 //
 // It only modifies *old if they are different. old must be non-nil.
 func Update[T EqualCloner[T]](old *T, new T) (changed bool) {
-	if new.Equal(*old) {
+	if (*old).Equal(new) {
 		return false
 	}
 	*old = new.Clone()