MAC/do: Use prison_lock()/prison_unlock()

Instead of fiddling directly with 'pr_mtx'.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47595

(cherry picked from commit 83fcbbff6b01ebbd1d8538cb5396d87d0a816db6)