Kernel: Widen PhysicalPage refcount to 32 bits

A 16-bit refcount is just begging for trouble right nowl.
A 32-bit refcount will be begging for trouble later down the line,
so we'll have to revisit this eventually. :^)

Kernel/VM/PhysicalPage.cpp

@@ -46,7 +46,7 @@ void PhysicalPage::return_to_freelist() &&
 
  InterruptDisabler disabler;
 
- m_retain_count = 1;
+ m_ref_count = 1;
 
  if (m_supervisor)
  MM.deallocate_supervisor_physical_page(move(*this));

Kernel/VM/PhysicalPage.h

@@ -42,14 +42,14 @@ class PhysicalPage {
 
  void ref()
  {
- ASSERT(m_retain_count);
- ++m_retain_count;
+ ASSERT(m_ref_count);
+ ++m_ref_count;
  }
 
  void unref()
  {
- ASSERT(m_retain_count);
- if (!--m_retain_count) {
+ ASSERT(m_ref_count);
+ if (!--m_ref_count) {
  if (m_may_return_to_freelist)
  move(*this).return_to_freelist();
  delete this;
@@ -58,7 +58,7 @@ class PhysicalPage {
 
  static NonnullRefPtr<PhysicalPage> create(PhysicalAddress, bool supervisor, bool may_return_to_freelist = true);
 
- u16 ref_count() const { return m_retain_count; }
+ u32 ref_count() const { return m_ref_count; }
 
  bool is_shared_zero_page() const;
 
@@ -68,7 +68,7 @@ class PhysicalPage {
 
  void return_to_freelist() &&;
 
- u16 m_retain_count { 1 };
+ u32 m_ref_count { 1 };
  bool m_may_return_to_freelist { true };
  bool m_supervisor { false };
  PhysicalAddress m_paddr;