mmu: correctly calculate size for unmap operations

Right now, we graciously accept - as does Linux - a mmap call for a size
smaller than a page: we align it up, and serve it. But the same alignment is
missing from unmap: so if the user rightfully tries to unmap using the same
size, it will fail.

The following test program succeeds on Linux but fails on OSv:

    int main () {
        void *ret = mmap(NULL, 64, PROT_READ, MAP_ANON | MAP_PRIVATE, -1, 0);
        if (ret == NULL) {
            return 1;
        }
        return munmap(ret, 64);
    }

After this patch, it works for us as well.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>