Use of the ZERO_COPY_SOCKETS options can result in an unusual state that
vm_object_backing_scan() was not written to handle. Specifically, a wired
page within a backing object that is shadowed by a page within the shadow
object. Handle this state by removing the wired page from the backing
object. The wired page will be freed by socow_iodone().

Stop masking errors: If a page is being freed by vm_object_backing_scan(),
assert that it is no longer mapped rather than quietly destroying any
mappings.

Tested by: Harald Schmalzbauer