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/PhysicalPage.h

@@ -64,6 +64,7 @@ public:
     u32 ref_count() const { return m_ref_count.load(AK::memory_order_consume); }
 
     bool is_shared_zero_page() const;
+    bool is_lazy_committed_page() const;
 
 private:
     PhysicalPage(PhysicalAddress paddr, bool supervisor, bool may_return_to_freelist = true);