squashfs: Fix sqfsls errors when root is a ldir

Previously, if root had more than 256 files or otherwise needed to be an
ldir, sqfsls would emit the error 'Inode not found.' which was caused by
code in sqfs_search_dir assuming it was a regular directory inode.

Signed-off-by: Campbell Suter <campbell@snapit.group>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>