wgengine/monitor: don't spam about Linux RTM_NEWRULE events

The earlier 2ba36c294b51b672073a36a599a61c758d0ffafa started listening
for ip rule changes and only cared about DELRULE events, buts its subscription
included all rule events, including new ones, which meant we were then
catching our own ip rule creations and logging about how they were unknown.

Stop that log spam.

Updates #1591

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

wgengine/monitor/monitor_linux.go

@@ -119,6 +119,9 @@ func (c *nlConn) Receive() (message, error) {
 			Dst:     dst,
 			Gateway: gw,
 		}, nil
+	case unix.RTM_NEWRULE:
+		// Probably ourselves adding it.
+		return ignoreMessage{}, nil
 	case unix.RTM_DELRULE:
 		// For https://github.com/tailscale/tailscale/issues/1591 where
 		// systemd-networkd deletes our rules.