f2fs: introduce F2FS_FEATURE_LOST_FOUND feature

This patch introduces a new feature, F2FS_FEATURE_LOST_FOUND, which
is set by mkfs. mkfs creates a directory named lost+found, which saves
unreachable files. If fsck finds a file which has no parent, or its
parent is removed by fsck, the file will be placed under lost+found
directory by fsck.

lost+found directory could not be encrypted. As a result, the root
directory cannot be encrypted too. So if LOST_FOUND feature is enabled,
let's avoid to encrypt root directory.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>