route: Fix compile warning with glibc

This fixes the following compile problem:
````
/route.c: In function 'rtnl_flush':
/route.c:45:15: error: ignoring return value of 'write' declared with attribute 'warn_unused_result' [-Werror=unused-result]
   45 |         (void)write(fd, "-1", 2);
      |               ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.
````

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

route.c

@@ -36,13 +36,16 @@ int route_table = 16800;
 
 static void rtnl_flush(void)
 {
+	ssize_t ret;
 	int fd;
 
 	fd = open("/proc/sys/net/ipv4/route/flush", O_WRONLY);
 	if (fd < 0)
 		return;
 
-	write(fd, "-1", 2);
+	ret = write(fd, "-1", 2);
+	if (ret != 2)
+		perror("write");
 	close(fd);
 }