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.
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

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