pfctl: Ensure parent queue is configured for FAIRQ

We failed to account for the FAIRQ scheduler in expand_altq(), which led
it to be set up without its parent queue.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30347

(cherry picked from commit dc784287967d45ab681dc51e9e20b78c8c535834)