sys_open-->open_namei-->path_walk
-->(vfs_create)
-->( error = -EISDIR; if (S_ISDIR(inode->i_mode) && (flag & FMODE_WRITE)))
-->dentry_open-->get_empty_filp
-->(get_write_access)
-->file_move
-->(f_op->open)
opendir 也是调用sys_open
