Kernel: Fix weird Region constructor that took nullable RefPtr<Inode>

It's never valid to construct a Region with a null Inode pointer using
this constructor, so just take a NonnullRefPtr<Inode> instead.

Kernel/VM/Region.cpp

@@ -17,7 +17,7 @@ Region::Region(const Range& range, const String& name, u8 access)
     MM.register_region(*this);
 }
 
-Region::Region(const Range& range, RefPtr<Inode>&& inode, const String& name, u8 access)
+Region::Region(const Range& range, NonnullRefPtr<Inode> inode, const String& name, u8 access)
     : m_range(range)
     , m_vmobject(InodeVMObject::create_with_inode(*inode))
     , m_name(name)

Kernel/VM/Region.h

@@ -131,7 +131,7 @@ public:
     // NOTE: These are public so we can make<> them.
     Region(const Range&, const String&, u8 access);
     Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmo, const String&, u8 access);
-    Region(const Range&, RefPtr<Inode>&&, const String&, u8 access);
+    Region(const Range&, NonnullRefPtr<Inode>, const String&, u8 access);
 
 private:
     Bitmap& ensure_cow_map() const;