If we are inserting a run of pages into a VM object and fail at some
point due to a memory allocation failure, we have to free all of the
pages in the run. We do that by resetting some fields and calling
vm_page_free_toq() on each page; this removes the page from the object
and frees it back to the buddy allocator.
If the page is supposed to be wired, we reset the reference count, but
this was done incorrectly: the VPRC_OBJREF flag must be retained as the
page still belongs to an object. Resetting it to zero will cause a
panic in vm_page_free_prep(): vm_page_free_object_prep() will subtract
VPRC_OBJREF from the refcount, causing underflow, and
vm_page_free_prep() subsequently calls panic() if the refcount is
non-zero.