network/netdev: enter ready state only when it is created by us

network/netdev: enter ready state only when it is created by us

Follow-up for PR #34909.

This fixes an issue that network interfaces cannot join a master netdev,
like bond or bridge, when the corresponding .netdev is reloaded.

With PR #34909, networkd supports reloading .netdev files. However,
When a .netdev file is modified and reloaded, ifindex is copied from
the old NetDev object to the new one. Thus, even if the interface is
successfully updated, netdev_set_ifindex_impl() will return 0 and
netdev_enter_ready() will never called. If the netdev is a kind of
master netdev, then port interfaces cannot join the master netdev,
as REQUEST_TYPE_SET_LINK_MASTER requires that the master netdev is
in the ready state.