mount: make sure mount_add_extras() is always invoked when we load a mount unit

mount: make sure mount_add_extras() is always invoked when we load a mount unit

We need to make sure that the slice property is initialized whenever
mount_load() is invoked, even if we fail to load things properly off
disk. This is important since we generally don't allow changing the
slice after a unit has been started. But given that we must track the
state of external objects with mount units we must hence initialize the
property no matter what.