Kernel: Implement lazy committed page allocation

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.

Kernel/VM/ContiguousVMObject.cpp

@@ -58,7 +58,7 @@ ContiguousVMObject::~ContiguousVMObject()
 {
 }
 
-NonnullRefPtr<VMObject> ContiguousVMObject::clone()
+RefPtr<VMObject> ContiguousVMObject::clone()
 {
     ASSERT_NOT_REACHED();
 }