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>