By designating a committed page pool we can guarantee to have physical
pages available for lazy allocation in mappings. However, when forking
we will overcommit. The assumption is that worst-case it's better for
the fork to die due to insufficient physical memory on COW access than
the parent that created the region. If a fork wants to ensure that all
memory is available (trigger a commit) then it can use madvise.
This also means that fork now can gracefully fail if we don't have
enough physical pages available.
We now have PrivateInodeVMObject and SharedInodeVMObject, corresponding
to MAP_PRIVATE and MAP_SHARED respectively.
Note that PrivateInodeVMObject is not used yet.
