shell32: Use FIELD_OFFSET instead of hand coding its functionality.

dlls/shell32/shfldr_netplaces.c

diff --git a/dlls/shell32/shfldr_netplaces.c b/dlls/shell32/shfldr_netplaces.c
index d0e9833..c661c51 100644
--- a/dlls/shell32/shfldr_netplaces.c
+++ b/dlls/shell32/shfldr_netplaces.c
@@ -66,8 +66,8 @@
 static const IPersistFolder2Vtbl vt_NP_PersistFolder2;
 
 
-#define _IPersistFolder2_Offset ((int)(&(((IGenericSFImpl*)0)->lpVtblPersistFolder2)))
-#define _ICOM_THIS_From_IPersistFolder2(class, name) class* This = (class*)(((char*)name)-_IPersistFolder2_Offset);
+#define _ICOM_THIS_From_IPersistFolder2(class, name) class* This = \
+    (class*)(((char*)name) - FIELD_OFFSET(IGenericSFImpl, lpVtblPersistFolder2))
 
 #define _IUnknown_(This)	(IUnknown*)&(This->lpVtbl)
 #define _IShellFolder_(This)	(IShellFolder*)&(This->lpVtbl)