network: do not regenerate MAC address if already set by userspace

network: do not regenerate MAC address if already set by userspace

When MACAddress= is unspecified, a stable MAC is generated to seed a newly
created netdev. Since existing netdevs are reconfigured on reload/restart,
this seed got re-applied to an already existing interface, conflicting with
an explicit MACAddress= from the matching .network file and flipping MAC.

Now, when MACAddress= is unspecified, only generate a new address if
addr_assign_type is not NET_ADDR_SET (i.e. not already set by userspace). On
first creation the interface does not exist yet, so this is a no-op and the
address is generated as before. Mirrors src/udev/net/link-config.c.

Fixes: #42457