btrfs-progs: kerncompat: update definition of container_of

Copy linux.git/include/linux/container_of.h definition of container_of
and the const variant (currently unused)

Signed-off-by: David Sterba <dsterba@suse.com>

include/kerncompat.h

@@ -456,9 +456,39 @@ do {					\
 
 #define IS_ENABLED(c) 0
 
+/* Are two types/vars the same type (ignoring qualifiers)? */
+#define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
+
+#define typeof_member(T, m)	typeof(((T*)0)->m)
+
+/**
+ * container_of - cast a member of a structure out to the containing structure
+ * @ptr:	the pointer to the member.
+ * @type:	the type of the container struct this is embedded in.
+ * @member:	the name of the member within the struct.
+ *
+ * WARNING: any const qualifier of @ptr is lost.
+ */
 #define container_of(ptr, type, member) ({				\
-        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
+	void *__mptr = (void *)(ptr);					\
-	        (type *)( (char *)__mptr - offsetof(type,member) );})
+	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
+		      __same_type(*(ptr), void),			\
+		      "pointer type mismatch in container_of()");	\
+	((type *)(__mptr - offsetof(type, member))); })
+
+/**
+ * container_of_const - cast a member of a structure out to the containing
+ *			structure and preserve the const-ness of the pointer
+ * @ptr:		the pointer to the member
+ * @type:		the type of the container struct this is embedded in.
+ * @member:		the name of the member within the struct.
+ */
+#define container_of_const(ptr, type, member)				\
+	_Generic(ptr,							\
+		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
+		default: ((type *)container_of(ptr, type, member))	\
+	)
+
 #ifndef __bitwise
 #ifdef __CHECKER__
 #define __bitwise __bitwise__