wgengine/tstun: also support DropSilently on PostFilterIn

Not a problem (yet). But should be consistent with other places that support both
types of drops.

wgengine/tstun/tun.go

@@ -346,8 +346,8 @@ func (t *TUN) filterIn(buf []byte) filter.Response {
 	}
 
 	if t.PostFilterIn != nil {
-		if t.PostFilterIn(p, t) == filter.Drop {
+		if res := t.PostFilterIn(p, t); res.IsDrop() {
-			return filter.Drop
+			return res
 		}
 	}