mkfs.f2fs: create lost+found directory

mkfs.f2fs: create lost+found directory

This patch introduces a new feature F2FS_FEATURE_LOST_FOUND. It can be
switched on by indicating `-O lost_found'. If LOST_FOUND feature is
enabled, an empty directory lost+found is created by mkfs.

This is a preparation for fsck. During fsck, the directory is used to
save unreachable files, which have no parent directory or their parent
directory is removed by fsck. Encrypted files are also allowed to be
saved here.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: use uint64_t to avoid build warning]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>