ip_divert: calculate delayed checksum for IPv6 adress family

Before passing an IPv6 packet to application apply delayed checksum
calculation. Mbuf flags will be lost when divert listener will return a
packet back, so we will not be able to do delayed checksum calculation
later. Also an application will get a packet with correct checksum.

Reviewed by: donner
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32807