network: wait until the DSA master interface becomes up

This is for the DSA subsystem, which have several stacked interfaces
on the master interface. To bring up a stacked interface, it is necessary
that the master is already up. See
https://github.com/systemd/systemd/issues/7478#issuecomment-348508263.

Note this is not necessary for newer kernels which includes
https://github.com/torvalds/linux/commit/9d5ef190e5615a7b63af89f88c4106a5bc127974.

Fixes #7478.