jail: fix a regression that creates zombies when removing dying jails

When adding jail descriptors, I split sys_jail remove in two, and
didn't properly track jail held between them when a jail was dying.
This fixes that as well as cleaning up the logic behind it.

PR: 290217
Reported by: David 'equinox' Lamparter <equinox at diac24.net>
Reviewed by: markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53200