swap_pager: Convert swap-space-full flags to bools

No functional change intended.

Reviewed by: alc, kib
MFC after: 2 weeks
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51321

(cherry picked from commit 5c76e9f4579677482b0f96d4b3581f5e1ea2e072)