Improve vm_object_scan_all_shadowed() to also check swap backing objects.

As noted in the removed comment, it is possible and not prohibitively
costly to look up the swap blocks for the given page index. Implement
a swap_pager_find_least() function to do that, and use it to iterate
simultaneously over both backing object page queue and swap
allocations when looking for shadowed pages.

Testing shows that number of new succesful scans, enabled by this
addition, is small but non-zero. When worked out, the change both
further reduces the depth of the shadow object chain, and frees unused
but allocated swap and memory.

Suggested and reviewed by: alc
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks