Ignore IPv6 NA and drop IPv6 NS when BACKUP CARP address is used

When system acts as CARP BACKUP ignore received IPv6 Neighbor Advertisements
to ensure that neighbor cache will not be changed.
Also do not send IPv6 Neighbor Solicitation from CARP BACKUP source address.
Such packets can confuse network switch and it detects MAC addresses
flapping.

Obtained from: Yandex LLC
MFC after: 2 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D36649