jail: Don't allow jails under dying parents

jail: Don't allow jails under dying parents

If a jail is created with jail_set(...JAIL_DYING), and it has a parent
currently in a dying state, that will bring the parent jail back to
life.  Restrict that to require that the parent itself be explicitly
brought back first, and not implicitly created along with the new
child jail.

Differential Revision:	https://reviews.freebsd.org/D28515