Inherit IPv6 checksum offloading flags to vlan interfaces.

if_vlan(4) interfaces inherit IPv4 checksum offloading flags from the
parent when VLAN_HWCSUM and VLAN_HWTAGGING flags are present on the
parent interface. Do the same for IPv6 checksum offloading flags.

Reported by: Harry Schmalzbauer
Reviewed by: np, gnn
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D10356