Don't preserve the page object/pindex in vm_page_insert_after()

[markj]

Summary

Per the KASSERT at the beginning of the function, we expect that the
page does not belong to any object, so its object and pindex fields are
meaningless. Reset them in the rare case that vm_radix_insert() fails.
This is just a micro-optimization.

I note that we do not clear the pindex field when freeing a page:
vm_page_remove() only clears the object field. But resetting it here
makes the intent more clear and doesn't hurt.

[kib]

I am curious why the code tried to preserve the previous object and pindex, as if the page could be returned to the cache (but it never is).