vm_object: speed up page collect flush

vm_pageout_cluster prepares an array for passing to vm_pageout_flush
by starting in the middle of a double-sized array and working out from
the middle. Using the same technique in vm_object_page_collect_flush
saves one loop that traverses a piece of linked list, and 80 bytes of
amd64 binary code.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46173