Redo r242889, now using the method from projects/amd64_xen_pv r240747.

Reminded by:	kib
MFC after:	3 days

sys/dev/xen/netback/netback_unit_tests.c

@@ -58,11 +58,11 @@ __FBSDID("$FreeBSD$");
 #define	TOSTRING(x) STRINGIFY(x)
 
 /**
- * Writes an error message to buffer if cond is false, and returns true
- * iff the assertion failed.  Note the implied parameters buffer and
+ * Writes an error message to buffer if cond is false
+ * Note the implied parameters buffer and
  * buflen
  */
-#define	XNB_ASSERT(cond) do {						\
+#define	XNB_ASSERT(cond) ({						\
 	int passed = (cond);						\
 	char *_buffer = (buffer);					\
 	size_t _buflen = (buflen);					\
@@ -71,7 +71,7 @@ __FBSDID("$FreeBSD$");
 		strlcat(_buffer, ":" TOSTRING(__LINE__) 		\
 		  " Assertion Error: " #cond "\n", _buflen);		\
 	}								\
-	} while (0)
+	})
 
 
 /**