vm_object: use lookup_range in page_clean_flush

In vm_object_page_clean_flush, replace the loop that uses
vm_radix_iter_next to visit consecutive pages with a call to
vm_radix_iter_lookup_range, then a loop to check the pages looked-up.
This achieves a small performance improvement.

Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D50425