mount: Disallow mounting over a jail root
Discussed with: jamie
Approved by: re (so, implicit)
Approved by: so
Security: CVE-2020-25584
Security: FreeBSD-SA-21:10.jail_mount
(cherry picked from commit 2425f5e9128102c8e6e473567ad6759a55be5b02)
(cherry picked from commit 3ae17faa370491d7ce1fcfc4d5b9cd1ed0117b67)