Reset kvo_laundry to prevent it being cumulative

sysctl vm.objects, used by vmstat -o was not resetting the
laundry counter to 0 inside the loop, causing it to show the sum of
the laundry count for all previous objects instead of only the laundry
count for the current object.

Fixes: a86373bc93ee1c850943e8585d0d426479378145
Reviewed by: markj
Sponsored by: Klara, Inc.
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D50907