Fix ticks wrap issue of lowmem test in vm_pageout_scan

Reviewed by:	jhb (D818)
MFC after:	3 days
Sponsored by:	Multiplay


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272071 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f