netns: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/225
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.htmlSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/YzIvfGXxfjdXmIS3@workSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/netns/generic.h

@@ -33,7 +33,7 @@ struct net_generic {
 			struct rcu_head rcu;
 		} s;
 
-		void *ptr[0];
+		DECLARE_FLEX_ARRAY(void *, ptr);
 	};
 };