add exercise of lab5 spoc discussion

related_info/lab5/lab5-spoc-discuss/user/pgdir.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <ulib.h>
+
+int
+main(void) {
+    cprintf("I am %d, print pgdir.\n", getpid());
+    print_pgdir();
+    cprintf("pgdir pass.\n");
+    return 0;
+}
+