vm_object: Export the number of wired pages in vm_object_list_handler()

Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53008

(cherry picked from commit ffca0c44ffe84352f6fdd24862c070d348d783d4)