MFC jail: Improve locking when removing prisons

Change the flow of prison_deref() so it doesn't let go of allprison_lock
until it's completely done using it (except for a possible drop as part
of an upgrade on its first try).

Differential Revision: https://reviews.freebsd.org/D28458

(cherry picked from commit 6e1d1bfcac77603541706807803a198c6d954d7c)