[Bug 278265] `route -n monitor' is quiet when redirecting its output

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278265

            Bug ID: 278265
           Summary: `route -n monitor' is quiet when redirecting its
                    output
           Product: Base System
           Version: 14.0-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: oleg@pcbtech.ru

Hello

It seems to me that this is an error at /sbin/route

This works fine

[qemu] ~ $ route -n monitor &
[1] 1526
[qemu] ~ $ arp -d 10.0.2.2 ; ifconfig gif0 create ; ifconfig gif0 destroy
10.0.2.2 (10.0.2.2) deleted
22:26:45.838 PID 1527 delete neigh 10.0.2.2 state REACHABLE lladdr
52:55:0a:00:02:02 iface em0 
22:26:45.839 PID    0 add/repl neigh 10.0.2.2 state REACHABLE lladdr
52:55:0a:00:02:02 iface em0 
22:26:45.840 PID    0 add/repl iface iface#3 gif0 admin DOWN oper DOWN mtu 1280 
22:26:45.843 PID    0 add/repl iface iface#3 gif0 admin DOWN oper DOWN mtu 1280 
22:26:45.843 PID    0 add/repl iface iface#3 gif0 admin DOWN oper DOWN mtu 1280 
22:26:45.843 PID    0 add/repl iface iface#3 gif0 admin DOWN oper DOWN mtu 1280 
22:26:45.845 PID    0 delete iface iface#3 gif0 admin DOWN oper DOWN mtu 1280 

but

[qemu] ~ $ route -n monitor > temp.log &
[1] 1535
[qemu] ~ $ arp -d 10.0.2.2 ; ifconfig gif0 create ; ifconfig gif0 destroy
10.0.2.2 (10.0.2.2) deleted
[qemu] ~ $ ll temp.log 
-rw-r--r--  1 root wheel 0 Apr  8 22:26 temp.log

file is zero length, there's no output from `route -n monitor' if we redirect
it to file (or pipe within the script)

that's seems to be related w/ 14.0-RELEASE when monitor_nl() and NETLINK is
used by default as there's no such behaviour in 13.3-RELEASE

I've tested it on amd64 GENERIC (under qemu and on real hardware servers).
Found it while trying to fix wireguard wg-quick tool (see bug #275026)

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