mmu: map file mappings with clear ptes.

When one does not care about dirty bit in a page table it is beneficial
to map everything as dirty since it will relieve HW from setting the bit
on first access, but since now file mappings use dirty bit to reduce
write back during sync lets map them clean initially, but only if file
is shared since for private file mapping sync is disabled.

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