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.