SIFTR: Fix compilation with -DSIFTR_IPV6

A few pieces of the SIFTR code that are behind #ifdef SIFTR_IPV6 have
not been updated as APIs have changed, etc.

Reported by: Alexander Sideropoulos <Alexander.Sideropoulos@netapp.com>
Reviewed by: rscheff, lstewart
Sponsored by: NetApp
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D32698