Don't preserve the page's object linkage in vm_page_insert_after().

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.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6669