mmu: Make fill from fill_page support variable page sizes

Previously, fill only supported small-page-size chunks.  However, it's
possible to avoid calling fill multiple times simply by allowing
variable page sizes.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: Add pivot_root() system call

This adds a simple pivot_root() system call that works on mountpoints
and simply removes 'put_old' from mount list so that VFS doesn't know
about it and adds renames 'new_root' ->m_path to '/'.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
ramfs: unmount

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
ZFS root filesystem

Use the new pivot_root() functionality to switch to ZFS root filesystem
once OSv is up and running.

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