mmu: make map_file_page object reusable.

Currently map_file_page object can be used only once since its internal
state changes during mapping operation. For demand paging we do not want
to allocate/delete an object each time mapping is needed, so, to make
map_file_page object reusable, this patch resets its sate to a starting
one during finalize().

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