xen/include: import sizeof_field() macro from Linux stddef.h

Co-locate it with the definition of offsetof() (since this is also in stddef.h
in the Linux kernel source). This macro will be needed in a subsequent patch.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/xen/compiler.h

@@ -76,6 +76,14 @@
 
 #define offsetof(a,b) __builtin_offsetof(a,b)
 
+/**
+ * sizeof_field(TYPE, MEMBER)
+ *
+ * @TYPE: The structure containing the field of interest
+ * @MEMBER: The field to return the size of
+ */
+#define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))
+
 #if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 201112L
 #define alignof __alignof__
 #endif