[PATCH] don't mmap zero pages for /dev/zero in advance
In current implementation mmap_zero() immedeately mmaps
zero page to the whole /dev/zero vma and thus allocates/pollutes all pte's.
This is actually not required and can be dangerous if vma is very big.
For CPT we do not need such "optimization",
as we can set vm_ops on this vma and return zero page on demand.