Micro-optimize kmem_unback().

We can remove some unnecessary object radix tree lookups by using the
object memq to iterate over pages in the specified range. This does not,
however, eliminate the lookup needed in vm_page_free_toq() to remove each
tree entry.

Reviewed by: alc, kib (previous revision)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D11945