pf: Reduce the data returned in DIOCGETSTATESNV

This call is particularly slow due to the large amount of data it
returns. Remove all fields pfctl does not use. There is no functional
impact to pfctl, but it somewhat speeds up the call.

It might affect other (i.e. non-FreeBSD) code that uses the new
interface, but this call is very new, so there's unlikely to be any. No
releases contained the previous version, so we choose to live with the
ABI modification.

Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30944