[bugfix] disable htree for /lost+found

[WaberZhuang]

e2fsck reports HTREE directory errors on converted extfs images:

sudo e2fsck -nfv /dev/sda
...
Pass 2: Checking directory structure
Problem in HTREE directory inode 11: block #2 not referenced
Invalid HTREE directory inode 11 (/lost+found).  Clear HTree index?

The root cause is in photon::fs::make_extfs() -> mkdir_lost_found(), which calls ext2fs_expand_dir() twice to ensure at least 2 blocks:

for (int i = 0; i < 2; i++) {
    ret = ext2fs_expand_dir(fs, ino);
}

The first call creates a hash-indexed (HTREE) directory. However, ext2fs_expand_dir() is not designed to be called again on an HTREE directory -- HTREE growth uses a separate implementation path. The second call corrupts the directory structure by failing to properly extend the hash tree.

Fix this by disabling HTREE for the lost+found directory when opening the filesystem.