add lab answers

labcodes_answer/lab7_result/user/badsegment.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <ulib.h>
+
+/* try to load the kernel's TSS selector into the DS register */
+
+int
+main(void) {
+    asm volatile("movw $0x28,%ax; movw %ax,%ds");
+    panic("FAIL: T.T\n");
+}
+