Handle a race of collapse with a retrying fault.

Both vm_object_scan_all_shadowed() and vm_object_collapse_scan() might
observe an invalid page left in the default backing object by the
fault handler that retried. Check for the condition and refuse to collapse.

Reported and tested by: pho
Reviewed by: jeff
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D23331