networkd: Add check to ensure link is down before attempting to enslave (#5853)

netdev to bond.

There are situations where a link can be in an "UP" state when
 systemd-networkd attempts to add the link to a bond device.
This is a problem because the bonding driver will refuse to
 enslave a link if it is in the "UP" state.
This check ensures systemd-networkd sets the link to "DOWN"
 before attempting to add the link to the bond.

Fixes #5838.

src/network/netdev/netdev.c

@@ -28,6 +28,7 @@
 #include "network-internal.h"
 #include "netdev/netdev.h"
 #include "networkd-manager.h"
+#include "networkd-link.h"
 #include "siphash24.h"
 #include "stat-util.h"
 #include "string-table.h"
@@ -221,6 +222,13 @@ static int netdev_enslave_ready(NetDev *netdev, Link* link, sd_netlink_message_h
         assert(link);
         assert(callback);
 
+        if (link->flags & IFF_UP) {
+                log_netdev_debug(netdev, "Link '%s' was up when attempting to enslave it. Bringing link down.", link->ifname);
+                r = link_down(link);
+                if (r < 0)
+                        return log_netdev_error_errno(netdev, r, "Could not bring link down: %m");
+        }
+
         r = sd_rtnl_message_new_link(netdev->manager->rtnl, &req, RTM_SETLINK, link->ifindex);
         if (r < 0)
                 return log_netdev_error_errno(netdev, r, "Could not allocate RTM_SETLINK message: %m");