Pull up following revision(s) (requested by ozaki-r in ticket #458):
sys/sys/psref.h: 1.3
sys/kern/subr_psref.c: 1.8-1.9
Fix psref(9) part of PR kern/52515. It is complete to fix the PR now.
implementated by ozaki-r@n.o, reviewed by riastradh@n.o, thanks.
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Improve debugging functions
- Make psref_check_duplication check just if a given psref is on the list
  - It checked both psref and target
  - Suggested by riastradh@ some time ago
- Add psref_check_existence that checks a releasing psref is surely on the list

(snj)