Enqueue lladdr_task to update link level address of vlan, when its parent
interface has changed.

During vlan reconfiguration without destroying interface, it is possible,
that parent interface will be changed. This usually means, that link
layer address of vlan will be different. Therefore we need to update all
associated with vlan's addresses permanent llentries - NDP for IPv6
addresses, and ARP for IPv4 addresses. This is done via lladdr_task
execution. To avoid extra work, before execution do the check, that L2
address is different.

No objection from: network
Obtained from: Yandex LLC
MFC after: 1 week
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D22243