pf: align IPv6 dummynet handling with IPv4

In e5c4987e3f we fixed issues with nat and dummynet, but only changed
the IPv4 code. Make the same change for IPv6 as well.

Reviewed by: glebius
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33086