eal: ensure constness of container_of target

This adds a check to ensure that the container_of() macro is not used to
cast away (remove) constness.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

lib/librte_eal/common/include/rte_common.h

@@ -348,6 +348,8 @@ rte_bsf32(uint32_t v)
 #ifndef container_of
 #define container_of(ptr, type, member)	__extension__ ({		\
 			const typeof(((type *)0)->member) *_ptr = (ptr); \
+			__attribute__((unused)) type *_target_ptr =	\
+				(type *)(ptr);				\
 			(type *)(((uintptr_t)_ptr) - offsetof(type, member)); \
 		})
 #endif