Delay the recursive decrement of pr_uref when jails are made invisible
but not removed; decrement it instead when the child jail actually
goes away. This avoids letting the counter go below zero in the case
where dying (pr_uref==0) jails are "resurrected", and an associated
KASSERT panic.

Submitted by: Steven Hartland
Approved by: re (bz)
MFC after: 1 week