networkd: fix link_up() (#12505)

Fillup IFLA_INET6_ADDR_GEN_MODE while we do link_up.

Fixes the following error:
```
dummy-test: Could not bring up interface: Invalid argument
```

After reading the kernel code when we do a link up
```
net/core/rtnetlink.c
IFLA_AF_SPEC
 af_ops->set_link_af(dev, af);
  inet6_set_link_af
   if (tb[IFLA_INET6_ADDR_GEN_MODE])
             Here it looks for IFLA_INET6_ADDR_GEN_MODE
```
Since link up we didn't filling up that it's failing.

Closes #12504.