mmu: pass fault type and shared status to fd->get_page() return cow status

File page allocator needs to know fault type (read/write) and mapping
type (shared/private) to handle page allocation correctly. It also needs
a way to communicate to a caller that some pages need to be mapped RO
for COW. This patch adds required functionality without using it yet.

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