mmu: implement madvise for MADV_DONTNEED

The jemalloc memory allocator will make intense use of MADV_DONTNEED to flush
pages it is no longer using. Respect that advice.

Let's keep returning -1 for the remaining cases so we don't fool anybody

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>