mmu: fix madvice(MADV_DONTNEED)

Fix madvice(MADV_DONTNEED) to use correct page ops for a VMA. Don't try
to use what we have today on mmaped files.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>