Skip Linux madvise(MADV_DONTNEED) on unmanaged objects.

vm_object_madvise() is a no-op for unmanaged objects, but we should also
limit the scope of mappings on which pmap_remove() is called. In
particular, with the WIP largepage shm objects patch the kernel must
remove mappings of such objects along superpage boundaries, and without
this check Linux madvise(MADV_DONTNEED) could violate that requirement.

Reviewed by: alc, kib
MFC with: r362631
Sponsored by: Juniper Networks, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D26084