Avoid page lookups in the top-level object in vm_object_madvise().

We can iterate over consecutive resident pages in the top-level object
using the object's page list rather than by performing lookups in the
object radix tree. This extends one of the optimizations in r312208 to the
case where a shadow chain is present.

Suggested by: alc
Reviewed by: alc, kib (previous version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D9282