wgengine/router: Increase range of rule priorities when detecting mwan3

Context: https://github.com/tailscale/tailscale/pull/5588#issuecomment-1260655929

It seems that if the interface at index 1 is down, the rule is not installed. As such,
we increase the range we detect up to 2004 in the hope that at least one of the interfaces
1-4 will be up.

Signed-off-by: Tom DNetto <tom@tailscale.com>

wgengine/router/router_linux.go

@@ -1713,7 +1713,7 @@ func checkOpenWRTUsingMWAN3() (bool, error) {
 		//
 		// We dont match on the mask because it can vary, or the
 		// table because I'm not sure if it can vary.
-		if r.Priority == 2001 && r.Mark != 0 {
+		if r.Priority >= 2001 && r.Priority <= 2004 && r.Mark != 0 {
 			return true, nil
 		}
 	}