pf: use 'struct ah' for the AH extension header rather than 'struct ip6_ext'

This fixes the build for NOINET6 kernels, but also more accurately reflects what
we're doing. The first two fields are the same, so the only functional change is
that we require slightly more data in the first fragment now.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D50658