For MACsec interfaces, kernel announces the parent ifindex in the
generic IFLA_LINK
netlink attribute, which we save in
NMPlatformLink.parent. There is no need to have a dedicate member in
NMPlatformLnkMacsec.
The dedicate member was never set and during a restart of NetworkManager the parent of the MACsec device could be unset leading to a failed assertion:
act_stage2_config: assertion 'parent' failed
Fixes: 85103656 ('platform: add support for macsec links')