dma-mbox-create: Restrict with Capsicum

The restriction here is pretty late and pretty minimal. We need a lot
of authority to open password databases, and don't do much after that
point.

Feedback from: lifanov at mail.lifanov.com (earlier version), emaste (earlier version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D7988