MAC/do: sysctl_rules(): Always copy the rules specification string

We are not guaranteed that the 'rules' storage stays stable if we don't
hold the prison lock. For this reason, always copy the specification
string (under the lock).

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47600