ck_cc: use __builtin_offsetof for CK_CC_CONTAINER on gcc-ish compilers

Less undefined behaviour is always good.  In practice, compilers seem
to handle either form correctly, but some static analysis tools have
trouble with (size_t)&((T *)0)->M).

include/ck_cc.h

@@ -50,6 +50,7 @@
  * Container function.
  * This relies on (compiler) implementation-defined behavior.
  */
+#ifndef CK_CC_CONTAINER
 #define CK_CC_CONTAINER(F, T, M, N)						\
 	CK_CC_INLINE static T *							\
 	N(F *p)									\
@@ -57,6 +58,7 @@
 		F *n = p;							\
 		return (T *)(void *)(((char *)n) - ((size_t)&((T *)0)->M));	\
 	}
+#endif
 
 #define CK_CC_PAD(x) union { char pad[x]; }
 

include/gcc/ck_cc.h

@@ -39,6 +39,15 @@
 #define CK_CC_UNUSED __attribute__((unused))
 #define CK_CC_USED   __attribute__((used))
 #define CK_CC_IMM "i"
+
+#define CK_CC_CONTAINER(F, T, M, N)					       \
+       CK_CC_INLINE static T *						       \
+       N(F *p)								       \
+       {								       \
+									       \
+	       return (T *)(void *)((char *)p - __builtin_offsetof(T, M));     \
+       }
+
 #if defined(__x86_64__) || defined(__x86__)
 #define CK_CC_IMM_U32 "Z"
 #define CK_CC_IMM_S32 "e"