jail: clean up locking around do_jail_attach

jail_attach_jd passed PD_DEREF to do_jail_attach, assuming it would take
care of freeing the held prison. This is not true, as do_jail_attach
immediately cleared that flag, leaving the jail stock in dying state
when it is later removed.

Reported by: markj
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D57674

(cherry picked from commit 3584cde63e416d150214192721cead4b735ca0b5)