useful constants: struct qstr for ".."

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -84,6 +84,8 @@ const struct qstr empty_name = QSTR_INIT("", 0);
 EXPORT_SYMBOL(empty_name);
 const struct qstr slash_name = QSTR_INIT("/", 1);
 EXPORT_SYMBOL(slash_name);
+const struct qstr dotdot_name = QSTR_INIT("..", 2);
+EXPORT_SYMBOL(dotdot_name);
 
 /*
  * This is the single most critical data structure when it comes