configfs: pin the symlink target's dirent instead of chasing ->ci_dentry

In the Linux kernel, the following vulnerability has been resolved: configfs: pin the symlink target's dirent instead of chasing ->ci_dentry create_link() reads the target's configfs_dirent from item->ci_dentry->d_fsdata, relying on the item reference taken by get_target(). That reference pins the item, not its dentry: the dentry is pinned by DCACHE_PERSISTENT, which configfs_remove_dir() releases via simple_rmdir() while the item is still alive. A symlink racing with rmdir of its target can therefore find ->ci_dentry freed and its dirent released, triggering WARN_ON(!atomic_read(&sd->s_count)) in configfs_get(). Take the dirent in get_target() as well, under ->d_lock and atomically with the item reference, and pass it down to create_link(). A hashed dentry has not been killed yet, so its ->d_fsdata reference keeps the dirent alive there.