swap_pager: move scan_all_shadowed, use iterators

Move vm_object_scan_all_shadowed from vm_object.c to swap_pager.c, and
rename it. In the moved function, use vm_page and swblk iterators to
advance through the objects. Avoid checking a backing page for
busyness or validity more than once, or when it is beyond the upper
bound of the scan.

Reviewed by: kib, markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D47150