batman-adv: Check type of x and y in seq_(before|after)

seq_before and seq_after depend on the fact that both sequence numbers
have the same type and thus the same bitwidth. We can ensure that by
compile time checking using a compare between the pointer to the
temporary buffers which were created using the typeof of both
parameters. For example gcc would create a warning like
"warning: comparison of distinct pointer types lacks a cast".
Signed-off-by: Sven Eckelmann <sven@narfation.org>

net/batman-adv/main.h

@@ -194,7 +194,10 @@ static inline int compare_eth(const void *data1, const void *data2)
  *  - before adding more than 127 to the starting value - it is a predecessor,
  *  - when adding 128 - it is neither a predecessor nor a successor,
  *  - after adding more than 127 to the starting value - it is a successor */
-#define seq_before(x, y) ({typeof(x) _dummy = (x - y); \
+#define seq_before(x, y) ({typeof(x) _d1 = (x); \
+			  typeof(y) _d2 = (y); \
+			  typeof(x) _dummy = (_d1 - _d2); \
+			  (void) (&_d1 == &_d2); \
 			  _dummy > smallest_signed_int(_dummy); })
 #define seq_after(x, y) seq_before(y, x)