vm_object: Reset kvo_path on each iteration in vm_object_list_handler()

Otherwise we print a bogus path for anonymous objects.

Approved by: re (cperciva)
Reviewed by: kib
MFC after: 1 week
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52997

(cherry picked from commit 13d866b314f87df901c0a1cbbbc56d56ee0d7b7c)
(cherry picked from commit 55ad7c573e5b7b8fa25dd368ccad1d1774c77824)