zdtm: calling futex via syscall saves error codes in errno

man 2 futex:
  In  the  event  of  an error (and assuming that futex() was invoked via
  syscall(2)), all operations return -1 and set  errno  to  indicate  the
  cause of the error.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/lib/lock.h

@@ -144,7 +144,9 @@ static void inline mutex_lock(mutex_t *m)
 
 	while ((c = atomic_inc(&m->raw))) {
 		ret = sys_futex(&m->raw, FUTEX_WAIT, c + 1, NULL, NULL, 0);
-		BUG_ON(ret < 0 && ret != -EWOULDBLOCK);
+		if (ret < 0)
+			pr_perror("futex");
+		BUG_ON(ret < 0 && errno != -EWOULDBLOCK);
 	}
 }